Stop local CI writing the shared mutable test image tag

The run built per-run images and then retagged them to fips-test:latest as
a compatibility bridge for the consumers that had never been migrated. The
bridge was the defect: while it existed, two concurrent runs shared one
binding, so a suite could start containers from the other run's binaries
and the verdict was recorded against a commit whose code never ran. It
fails silently by construction — the run is green either way, and nothing
compares a running container's binary against the commit under test.

Every consumer now reads the run's image, so the retag is deleted rather
than kept. That is the point of deleting it: a consumer that was missed
fails loudly instead of quietly resolving whichever run wrote the tag last.

Two consequences handled here. The cleanup script ran ip(8) inside
fips-test:latest to reap simulation interfaces, and nothing writes that tag
any more, so it now takes the caller's image, then the run's, then any
surviving test image — the last of which is what keeps an unscoped --reap
working, since that path execs before the run identity is exported. And a
guard checks statically that nothing names the shared tag, because on a
host with a hand-built copy lying around a reintroduction would run green;
it is break-checked against a reintroduced compose consumer and a
reintroduced default. Both runners gate on it, as they do the other guards.

.github/workflows/ci.yml

@@ -65,6 +65,8 @@ jobs:
         run: python3 testing/check-log-strings.py
       - name: Check no tested function's exit status is a log call's
         run: python3 testing/check-trailing-log.py
+      - name: Check nothing resolves the shared mutable test image
+        run: bash testing/check-image-scoping.sh
       # Hermetic: synthetic ping functions, no containers, ~45s. Lives beside
       # the other two so both runners gate on it identically — putting it in
       # only one would create exactly the drift check-ci-parity.sh exists to