refactor(corpus-seeded-mechanisms): apply review round 2 suggestions

- variant_search.search_tree skips a malformed store row with a
  variants_store_row_invalid degradation instead of aborting the scan;
  a hit on a demoted or flowless overlay record is never merged (test).
- seed.pair_lessons is the one shape derivation shared by the exporter
  and leave-one-out; labels pass line and sink together; pairs without a
  labeled function are reported as no_labeled_function.
- config parses [variants].max_mechanisms with the shared int helper.
- regress.candidate.hotspot_from_finding is the one zero-score hotspot
  helper for forced inventory and variant findings.
- design synced (SearchResult, hits_to_findings summaries, candidate.py).

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

src/openultrasast/regress/candidate.py

@@ -99,19 +99,24 @@ def _hotspot_for_finding(finding: StaticFinding, hotspots: Sequence[Hotspot]) ->
     return None
 
 
-def _hotspot_from_forced_finding(finding: StaticFinding) -> Hotspot:
+def hotspot_from_finding(finding: StaticFinding, *, rationale: str) -> Hotspot:
+    """A zero-score hotspot wrapping one finding: forced sev-5 inventory or a variant of a known mechanism."""
     return Hotspot(
         path=finding.path,
         function_name=finding.function_name,
         score=0.0,
         band="low",
         signals={},
-        rationale="forced reachable severity-5 inventory",
+        rationale=rationale,
         test_hint=None,
         inventory_finding_ids=(finding.finding_id,),
     )
 
 
+def _hotspot_from_forced_finding(finding: StaticFinding) -> Hotspot:
+    return hotspot_from_finding(finding, rationale="forced reachable severity-5 inventory")
+
+
 class RegressionRunner:
     """Run a snippet in the sandbox only after the structural safety check passes."""