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/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.