feat(corpus-seeded-mechanisms): the mechanisms lever runs from ousast improve

Review rounds 3-5 remediations (Req 5, 7).

- ousast improve resolves candidates from the target's calibration dir,
  then the working directory's (where `mechanisms export` writes), and
  admits into <target>/.openultrasast/calibration/mechanisms.jsonl,
  where a scan of that target reads; --mechanism-candidates,
  --mechanism-store, --no-mechanisms; --dry-run redirects the store to
  scratch; accepted rounds print admit/retract <id>.
- run_improvement threads the store paths; RoundOutcome carries the
  mechanism edits; reverted mechanism keys are blocked outright; a
  no-gain admission is rejected as mechanism_no_gain; a retraction may
  name an admitted record that left the candidate set.
- leave-one-out forwards variant-search degradations and pins the
  containment and label rules; mechanisms export defaults to the
  candidates file so it never re-admits a tombstone.
- design §Lever rewritten to the implemented gate; README and docs show
  export -> improve -> scan.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

src/openultrasast/improve/validator.py

@@ -118,13 +118,20 @@ class EvolveValidator:
         else:
             raise StrictValidationError(f"unknown edit type {type(edit).__name__}", kind="rule_change")
 
-    def validate_mechanism(self, edit: MechanismEdit, candidates: Mapping[str, object]) -> None:
-        """Req 5.2: only records the exporter derived from trusted pairs, with a closed guard and identifier-only shape text."""
+    def validate_mechanism(
+        self, edit: MechanismEdit, candidates: Mapping[str, object], admitted: Mapping[str, object] | None = None
+    ) -> None:
+        """Req 5.2: only records the exporter derived from trusted pairs, with a closed guard and identifier-only shape text.
+
+        A retraction may also name a record that is admitted in the scan store but no longer in the candidate set.
+        """
         from ..semantic.variants import GUARD_KINDS, SOURCE_KINDS
 
         if edit.action not in MECHANISM_ACTIONS:
             raise StrictValidationError(f"mechanism edit action {edit.action!r} is not admit or retract", kind="mechanism_change")
         record = candidates.get(edit.mechanism_id)
+        if record is None and edit.action == "retract" and admitted is not None:
+            record = admitted.get(edit.mechanism_id)
         if record is None:
             raise StrictValidationError(
                 f"unknown mechanism {edit.mechanism_id!r}: not in the exporter candidate set", kind="mechanism_change"