dev: require the pinned toolchain, and clear the QEMU-era leftovers

The host gate accepted a WATCOM environment override and skipped when nothing
was found. Both are wrong for what it is: a system-wide Open Watcom is a
different version reporting different diagnostics, and a gate that skips is a
gate that is not running, which is the exact shape of the problem this file was
added to close. It now uses .dosboxx/watcom only and fails with the setup
command when that is absent, matching how dosboxx.py already behaves.

Nothing else in the project reaches for a system install: the DOS session sets
WATCOM=W: before calling BUILD.BAT, so the C:\DEVEL\WATCOMC fallback inside it
is unreachable.

fec/test-dos.bat was tracked but dead -- the runner generates RUN.BAT and only
copies build-dos.bat -- so it goes, along with the comment claiming it drives
the build and the three fixture READMEs that still pointed at it. The registry
decides what runs now.

(.qemu/ is untracked local debris from the same era and is left alone.)

fec/build-dos.bat

@@ -1,5 +1,5 @@
 @echo off
-rem Open Watcom C89 build. TEST-DOS.BAT runs this from C:\FEC.
+rem Open Watcom C89 build. The runner's generated RUN.BAT calls this from C:\FEC.
 C:
 cd \FEC
 if exist BUILD.OK del BUILD.OK

fec/test-dos.bat

@@ -1,288 +0,0 @@
-@echo off
-rem FreeDOS smoke tests. All work happens on the writable C: drive.
-C:
-cd \FEC
-if exist TEST.OK del TEST.OK
-if exist TEST.FAIL del TEST.FAIL
-call C:\FEC\BUILD.BAT
-if not exist BUILD.OK goto test_fail
-if "%WATCOM%"=="" set WATCOM=C:\DEVEL\WATCOMC
-if not exist %WATCOM%\BINW\WCL.EXE goto test_fail
-if not exist %WATCOM%\BINW\WCL386.EXE goto test_fail
-set PATH=%WATCOM%\BINW;%WATCOM%\BINP;%PATH%
-
-fec.exe --dump-ast TESTS\PASS\BASIC.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast TESTS\PASS\LITERALS.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast TESTS\PASS\KEYBUILT.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast TESTS\PASS\V012FORM.FE > nul
-if errorlevel 1 goto test_fail
-
-fec.exe --dump-ast STD\CORE.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\FMT.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\IO.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\LIST.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\MAP.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\MEM.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\STR.FE > nul
-if errorlevel 1 goto test_fail
-fec.exe --dump-ast STD\SYS.FE > nul
-if errorlevel 1 goto test_fail
-
-fec.exe --dump-ast TESTS\FAIL\MISSSEMI.FE > nul
-if not errorlevel 1 goto test_fail
-fec.exe --dump-ast TESTS\FAIL\UNCLCOMM.FE > nul
-if not errorlevel 1 goto test_fail
-fec.exe --dump-ast TESTS\FAIL\LOGICAL.FE > nul
-if not errorlevel 1 goto test_fail
-
-if exist TESTS\M2\HELLO.C del TESTS\M2\HELLO.C
-if exist TESTS\M2\HELLO.EXE del TESTS\M2\HELLO.EXE
-if exist TESTS\M2\SCOPES.C del TESTS\M2\SCOPES.C
-if exist TESTS\M2\SCOPES.EXE del TESTS\M2\SCOPES.EXE
-if exist TESTS\M2\CAST16.C del TESTS\M2\CAST16.C
-if exist TESTS\M2\CAST16.EXE del TESTS\M2\CAST16.EXE
-
-rem M2 bits32 path: Open Watcom 32-bit compiler and DOS extender executable.
-fec.exe --target=bits32 --emit-c TESTS\M2\HELLO.FE -o TESTS\M2\HELLO.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M2\HELLO.EXE TESTS\M2\HELLO.C
-if errorlevel 1 goto test_fail
-TESTS\M2\HELLO.EXE
-if errorlevel 1 goto test_fail
-
-fec.exe --target=bits32 --emit-c TESTS\M2\SCOPES.FE -o TESTS\M2\SCOPES.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M2\SCOPES.EXE TESTS\M2\SCOPES.C
-if errorlevel 1 goto test_fail
-TESTS\M2\SCOPES.EXE
-if errorlevel 1 goto test_fail
-
-rem M2 bits16 regression path remains on compiler A (wcl).
-fec.exe --target=bits16 --emit-c TESTS\M2\CASTWHIL.FE -o TESTS\M2\CAST16.C > nul
-if errorlevel 1 goto test_fail
-wcl -q -za -bt=dos -fe=TESTS\M2\CAST16.EXE TESTS\M2\CAST16.C
-if errorlevel 1 goto test_fail
-TESTS\M2\CAST16.EXE
-if errorlevel 1 goto test_fail
-
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-COND.FE -o TESTS\M2\BAD-CO.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-CAST.FE -o TESTS\M2\BAD-CA.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-ASGN.FE -o TESTS\M2\BAD-AS.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-UNK.FE -o TESTS\M2\BAD-UN.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-ARI.FE -o TESTS\M2\BAD-AR.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-TYPE.FE -o TESTS\M2\BAD-TY.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-RET.FE -o TESTS\M2\BAD-RE.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-UNIT.FE -o TESTS\M2\BAD-UI.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M2\BAD-VOID.FE -o TESTS\M2\BAD-VO.C > nul
-if not errorlevel 1 goto test_fail
-
-if exist TESTS\M3\STRUCT.C del TESTS\M3\STRUCT.C
-if exist TESTS\M3\STRUCT.EXE del TESTS\M3\STRUCT.EXE
-if exist TESTS\M3\ENUM.C del TESTS\M3\ENUM.C
-if exist TESTS\M3\ENUM.EXE del TESTS\M3\ENUM.EXE
-if exist TESTS\M3\ARRAY.C del TESTS\M3\ARRAY.C
-if exist TESTS\M3\ARRAY.EXE del TESTS\M3\ARRAY.EXE
-if exist TESTS\M3\STR.C del TESTS\M3\STR.C
-if exist TESTS\M3\STR.EXE del TESTS\M3\STR.EXE
-if exist TESTS\M3\FOR.C del TESTS\M3\FOR.C
-if exist TESTS\M3\FOR.EXE del TESTS\M3\FOR.EXE
-if exist TESTS\M3\NESTED.C del TESTS\M3\NESTED.C
-if exist TESTS\M3\NESTED.EXE del TESTS\M3\NESTED.EXE
-if exist TESTS\M3\CHAR.C del TESTS\M3\CHAR.C
-if exist TESTS\M3\CHAR.EXE del TESTS\M3\CHAR.EXE
-if exist TESTS\M3\ARRAYCTX.C del TESTS\M3\ARRAYCTX.C
-if exist TESTS\M3\ARRAYCTX.EXE del TESTS\M3\ARRAYCTX.EXE
-if exist TESTS\M3\BOUNDS.C del TESTS\M3\BOUNDS.C
-if exist TESTS\M3\BOUNDS.EXE del TESTS\M3\BOUNDS.EXE
-if exist TESTS\M3\BOUNDS-N.C del TESTS\M3\BOUNDS-N.C
-if exist TESTS\M3\BOUNDS-N.EXE del TESTS\M3\BOUNDS-N.EXE
-
-fec.exe --target=bits32 --emit-c TESTS\M3\STRUCT.FE -o TESTS\M3\STRUCT.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\STRUCT.EXE TESTS\M3\STRUCT.C
-if errorlevel 1 goto test_fail
-TESTS\M3\STRUCT.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\ENUM.FE -o TESTS\M3\ENUM.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\ENUM.EXE TESTS\M3\ENUM.C
-if errorlevel 1 goto test_fail
-TESTS\M3\ENUM.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\ARRAY.FE -o TESTS\M3\ARRAY.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\ARRAY.EXE TESTS\M3\ARRAY.C
-if errorlevel 1 goto test_fail
-TESTS\M3\ARRAY.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\MUTABLE.FE -o TESTS\M3\MUTABLE.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\MUTABLE.EXE TESTS\M3\MUTABLE.C
-if errorlevel 1 goto test_fail
-TESTS\M3\MUTABLE.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BAD-MLET.FE -o TESTS\M3\BAD-MLET.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BAD-SHWR.FE -o TESTS\M3\BAD-SHWR.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\STR.FE -o TESTS\M3\STR.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\STR.EXE TESTS\M3\STR.C
-if errorlevel 1 goto test_fail
-TESTS\M3\STR.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\FOR.FE -o TESTS\M3\FOR.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\FOR.EXE TESTS\M3\FOR.C
-if errorlevel 1 goto test_fail
-TESTS\M3\FOR.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\NESTED.FE -o TESTS\M3\NESTED.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\NESTED.EXE TESTS\M3\NESTED.C
-if errorlevel 1 goto test_fail
-TESTS\M3\NESTED.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\CHAR.FE -o TESTS\M3\CHAR.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\CHAR.EXE TESTS\M3\CHAR.C
-if errorlevel 1 goto test_fail
-TESTS\M3\CHAR.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\ARRAYCTX.FE -o TESTS\M3\ARRAYCTX.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\ARRAYCTX.EXE TESTS\M3\ARRAYCTX.C
-if errorlevel 1 goto test_fail
-TESTS\M3\ARRAYCTX.EXE
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BOUNDS.FE -o TESTS\M3\BOUNDS.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\BOUNDS.EXE TESTS\M3\BOUNDS.C
-if errorlevel 1 goto test_fail
-TESTS\M3\BOUNDS.EXE
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\SLCBOUND.FE -o TESTS\M3\SLCBOUND.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\SLCBOUND.EXE TESTS\M3\SLCBOUND.C
-if errorlevel 1 goto test_fail
-TESTS\M3\SLCBOUND.EXE
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --no-checks --emit-c TESTS\M3\BOUNDS.FE -o TESTS\M3\BOUNDS-N.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -bt=dos -fe=TESTS\M3\BOUNDS-N.EXE TESTS\M3\BOUNDS-N.C
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADFLD.FE -o TESTS\M3\BADFLD.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADMAT.FE -o TESTS\M3\BADMAT.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADARR.FE -o TESTS\M3\BADARR.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADCYCLE.FE -o TESTS\M3\BADCYCLE.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADSTR.FE -o TESTS\M3\BADSTR.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADCHAR.FE -o TESTS\M3\BADCHAR.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADFIELD.FE -o TESTS\M3\BADFIELD.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M3\BADINDEX.FE -o TESTS\M3\BADINDEX.C > nul
-if not errorlevel 1 goto test_fail
-
-if exist TESTS\M4\FORMAT.C del TESTS\M4\FORMAT.C
-if exist TESTS\M4\FORMAT.EXE del TESTS\M4\FORMAT.EXE
-fec.exe --target=bits32 --emit-c TESTS\M4\FORMAT.FE -o TESTS\M4\FORMAT.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -wx -wcd=202 -bt=dos -fe=TESTS\M4\FORMAT.EXE TESTS\M4\FORMAT.C
-if errorlevel 1 goto test_fail
-TESTS\M4\FORMAT.EXE > nul
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\TRY-FPR.FE -o TESTS\M4\TRY-FPR.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -wx -wcd=202 -bt=dos -fe=TESTS\M4\TRY-FPR.EXE TESTS\M4\TRY-FPR.C
-if errorlevel 1 goto test_fail
-TESTS\M4\TRY-FPR.EXE > nul
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\PROP.FE -o TESTS\M4\PROP.C > nul
-if errorlevel 1 goto test_fail
-wcl386 -q -za -wx -wcd=202 -bt=dos -fe=TESTS\M4\PROP.EXE TESTS\M4\PROPTEST.C
-if errorlevel 1 goto test_fail
-TESTS\M4\PROP.EXE > nul
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-ARI.FE -o TESTS\M4\BAD-ARI.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-VERB.FE -o TESTS\M4\BAD-VERB.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-RUN.FE -o TESTS\M4\BAD-RUN.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-TYPE.FE -o TESTS\M4\BAD-TYP.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-TRY.FE -o TESTS\M4\BAD-TRY.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-WRIT.FE -o TESTS\M4\BAD-WRI.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-BUFW.FE -o TESTS\M4\BAD-BUFW.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-MANY.FE -o TESTS\M4\BAD-MANY.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-OPEN.FE -o TESTS\M4\BAD-OPEN.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M4\BAD-CLS.FE -o TESTS\M4\BAD-CLS.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\DEFER.FE -o TESTS\M5\DEFER.C > nul
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\OWNED.FE -o TESTS\M5\OWNED.C > nul
-if errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-MOVE.FE -o TESTS\M5\BAD-MOVE.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-DEST.FE -o TESTS\M5\BAD-DES.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-DROP.FE -o TESTS\M5\BAD-DROP.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-DBL.FE -o TESTS\M5\BAD-DBL.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-COND.FE -o TESTS\M5\BAD-COND.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-PROJ.FE -o TESTS\M5\BAD-PROJ.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-CLOS.FE -o TESTS\M5\BAD-CLOS.C > nul
-if not errorlevel 1 goto test_fail
-fec.exe --target=bits32 --emit-c TESTS\M5\BAD-LOOP.FE -o TESTS\M5\BAD-LOOP.C > nul
-if not errorlevel 1 goto test_fail
-if exist TESTS\M5\RUNT-G.C del TESTS\M5\RUNT-G.C
-if exist TESTS\M5\RUNTIME.O del TESTS\M5\RUNTIME.O
-if exist TESTS\M5\RUNTIME.EXE del TESTS\M5\RUNTIME.EXE
-fec.exe --target=bits32 --emit-c TESTS\M5\RUNTIME.FE -o TESTS\M5\RUNT-G.C > nul
-if errorlevel 1 goto test_fail
-rem Compile generated source and the C89 runtime harness in one WCL386 invocation
-rem so both objects use the same DOS/4GW startup and runtime library.
-wcl386 -q -za -bt=dos -dmalloc=m5_malloc -dfree=m5_free -fe=TESTS\M5\RUNTIME.EXE TESTS\M5\RUNT-G.C TESTS\M5\RUNTIME.C
-if errorlevel 1 goto test_fail
-TESTS\M5\RUNTIME.EXE
-if errorlevel 1 goto test_fail
-
-echo OK>TEST.OK
-cd C:\FEC
-goto test_done
-
-:test_fail
-echo FAIL>TEST.FAIL
-verify other 2>nul
-
-:test_done

fec/tests/m6/README.md

@@ -4,7 +4,7 @@ These fixtures pin the M6 ownership/borrow rules before implementation.
 
 - `ok*.fe` must compile with `--target=bits32 --emit-c`.
 - `bad*.fe` must fail compilation; the first line follows the `// ERROR:<line>:<substring>` convention from SPEC §12.
-- They are intentionally not wired into `TEST-DOS.BAT` while `master` is M5, so adding these fixtures does not make the current milestone red.
+- They are registered in `src/ferrolang_vm/registry.py`, which is what decides whether a milestone runs.
 - When M6 starts, wire this directory into the DOS/QEMU gate without changing the expected result of any fixture.
 - M6 also owns the general-global borrow restriction from R10 because AGENTS.md explicitly groups that change with the `own.c` state-machine work.
 

fec/tests/m7/README.md

@@ -3,6 +3,6 @@
 M7 adds optionals and error unions on top of the M6 ownership model.
 
 `ok*.fe` must compile. `bad*.fe` must fail according to the first-line error marker.
-The files are not wired into `TEST-DOS.BAT` until M7 work begins.
+The files are registered in `src/ferrolang_vm/registry.py`.
 
 Coverage: contextual `null`, `?T`, `.?`, `Some`/`None` pattern-only non-destructive views, `mem.replace` extraction, lazy `orelse`/`catch`, nominal error unions, `try`, block/short `catch`, and error code/name uniqueness, plus R4/R7 interactions with optional references/owners.

fec/tests/m8/README.md

@@ -21,6 +21,6 @@ Fail cases:
 - `pubpriv`: a public signature cannot expose a private nominal type.
 - `errnom`: nominal error cannot flow into `core.Error` via `try`.
 
-The current M5 `TEST-DOS.BAT` is intentionally unchanged. M8 should add procedural checks for `.fei` creation/hash invalidation and deterministic `fe_errors.h` using these fixtures.
+They are not registered in `src/ferrolang_vm/registry.py` yet. M8 should add procedural checks for `.fei` creation/hash invalidation and deterministic `fe_errors.h` using these fixtures.
 
 The M8 DOS gate must additionally construct two separate `-I` roots containing the same canonical unit and require an ambiguity error; repeat the case with two paths to the same canonical file and require deduplication. It must also verify that `std.*` resolves only from the built-in std root, ordinary user units never do, and that changing a private non-generic implementation preserves the dependent interface-cache hit. Those checks need temporary roots/cache inspection and deliberately remain procedural rather than encoding host paths in fixtures.

tools/tests/test_host_syntax.py

@@ -17,13 +17,14 @@ What it still cannot see is the DOS environment itself -- memory limits, the
 command line length, the filesystem. The DOS build and the milestone suite
 remain the gate.
 
-Skipped when the toolchain has not been downloaded, so the suite runs anywhere.
+It uses the pinned toolchain only. There is no environment override and no
+skip: a system-wide Watcom is a different version reporting different things,
+and a gate that quietly skips is not a gate.
 """
 from __future__ import annotations
 
 import os
 import re
-import shutil
 import subprocess
 from pathlib import Path
 
@@ -37,23 +38,20 @@ SOURCES = ("arena", "diag", "lexer", "ast", "parser", "types", "m7", "own",
            "check", "lower", "emit_c", "driver")
 
 
-def _watcom() -> Path | None:
-    """Prefer the toolchain the runner pins over anything installed system-wide."""
-    root = os.environ.get("WATCOM")
-    candidates = [ROOT / ".dosboxx" / "watcom"]
-    if root:
-        candidates.append(Path(root))
-    for base in candidates:
-        if (base / "binnt" / "wcl.exe").is_file():
-            return base
-    return None
-
-
 @pytest.fixture(scope="session")
 def watcom() -> Path:
-    base = _watcom()
+    """The pinned toolchain, and nothing else.
-    if base is None:
+
-        pytest.skip("run `ferro-dos setup` to download the pinned toolchain")
+    Deliberately no environment override and no skip. A system-wide Open Watcom
+    is a different version with different diagnostics, and a gate that skips is
+    a gate that is not running -- which is the failure mode this file exists to
+    close. dosboxx.py fails the same way when the toolchain is missing.
+    """
+    base = ROOT / ".dosboxx" / "watcom"
+    if not (base / "binnt" / "wcl.exe").is_file():
+        raise AssertionError(
+            f"the pinned Open Watcom is not at {base}; "
+            "run `uv run ferro-dos setup --accept-watcom-license`")
     return base