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/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.