kind: infra workflow: v1

fu6 idempotency test leaks `neg_sp_police` / `neg_sp_ph4` into the global CONTEXTS registry at runtime (no teardown pop)

Goal

Stop tests/test_issue1090_fu6.py from leaving two keys in the process-global CONTEXTS registry after it runs, so a later-running test module no longer inherits a poisoned registry view. This is the RUNTIME sibling of the collection-time contamination #2217 fixed; #2217 deliberately scoped it out.

Evidence

Reproduces on pristine main — this is not introduced by #2217. Independently confirmed pre-existing by the Codex reviewer's own git probes during #2217 round 1.
Mechanism, at tests/test_issue1090_fu6.py:780 (test_register_capture_contexts_idempotent_and_foreign_binding_refusal): the test calls fu6._register_capture_contexts(), which writes neg_sp_police and neg_sp_ph4 into the global CONTEXTS. Its finally block restores only the ONE key it deliberately shadowed (CONTEXTS["neg_sp_police"] = old) — it never POPS the two keys the registration ADDED. So both keys outlive the test for the rest of the process.
Observed consequence: the explicit order fu6 → test_artifacts_organisms fails one test. Under any other order it passes, which is what makes it a latent order-dependence rather than a standing red.
Not caught by #2217's guard by design: that guard watches COLLECTION-time registry growth (tests/conftest.py hooks + tests/test_no_import_time_registry_mutation.py). This leak happens at RUNTIME, after collection is finished.

Suggested approach

Apply the registry_hygiene fixture that #2217 moved into tests/conftest.py — it snapshots CONTEXTS / NEGATIVE_PANELS and pops every key added during the test, which is exactly this defect's shape. Prefer the shared fixture over a hand-rolled finally pop so the next test with the same shape inherits the protection.

Check the rest of tests/test_issue1090_fu6.py for sibling registration sites while in there — the file has more than one _register_capture_contexts() call — and apply the fixture at the right scope rather than per-test if several tests share the leak.

Acceptance criteria

The explicit order fu6 → test_artifacts_organisms passes.
After the fu6 module runs, CONTEXTS contains neither neg_sp_police nor neg_sp_ph4 (assert it, so the fix cannot silently regress).
The existing fu6 assertions — idempotency and the loud foreign-binding refusal — still hold; the fixture must not paper over the ValueError the test requires.
Full-suite run stays green.

Provenance

Carried action item (2) recorded on #2217 before its Step 10d merge, filed after that merge landed. Raised during #2217 round 1 as concern preexisting-fu6-runtime-registry-leak (severity CONCERN, raised by the implementer). Filing was deliberately sequenced AFTER #2217's merge because the suggested fix applies a fixture that #2217 was concurrently moving into tests/conftest.py — filing earlier would have put two sessions on the same file (.claude/rules/cross-session-writer-arbitration.md).