feat: require validation failure history in review reports (Closes #396)

Reviewer runs that observe a validation failure and later rerun to green
must report the failure and its investigation; a later pass never erases
the earlier failure:

- New reviewer_validation_failure_history.py verifier: reports must carry
  a Validation failure history section naming every observed failure with
  command, failing test or error, suspected cause, reproduced status,
  baseline master status, and PR-caused evidence; rerun-pass claims must
  explain what changed between runs, whether environmental state (such as
  stale /tmp files) was cleaned, and why the pass is trustworthy; an
  unexplained transient failure cannot be reported as a bare pass and
  must use the status 'passed after transient failure investigation'
  (exported as TRANSIENT_INVESTIGATION_STATUS).
- Re-export from review_proofs and wire into build_final_report as a
  downgrade check.
- Review-merge final-report schema gains the Validation failure history
  template section; contract test locks the markers and an export test
  locks the re-export.
- 12 tests covering the issue's scenarios: failing-then-passing rerun,
  baseline-equivalent failure, /tmp state contamination, unexplained
  transient failure, and a report omitting an earlier failure.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

review_proofs.py

@@ -1735,6 +1735,11 @@ def build_final_report(checkout_proof, inventory, validation, contamination,
         if report_text
         else {"proven": True, "block": False, "reasons": []}
     )
+    validation_failure_history = (
+        assess_validation_failure_history_report(report_text)
+        if report_text
+        else {"proven": True, "block": False, "reasons": []}
+    )
     if baseline_validation is None and report_text:
         baseline_validation = assess_reviewer_baseline_validation_proof(
             report_text=report_text,
@@ -1932,6 +1937,11 @@ def build_final_report(checkout_proof, inventory, validation, contamination,
             "inventory pagination or worktree fields inconsistent (#293)"
         )
         downgrade_reasons.extend(inventory_worktree.get("reasons", []))
+    if not validation_failure_history.get("proven"):
+        downgrade_reasons.append(
+            "validation failure history incomplete or omitted (#396)"
+        )
+        downgrade_reasons.extend(validation_failure_history.get("reasons", []))
     if not baseline_validation.get("proven"):
         downgrade_reasons.append(
             "reviewer baseline validation proof missing or failed (#325)"
@@ -5425,6 +5435,15 @@ def assess_inventory_worktree_report(report_text, **kwargs):
     return _assess(report_text, **kwargs)
 
 
+def assess_validation_failure_history_report(report_text, **kwargs):
+    """#396: observed validation failures must be reported with investigation."""
+    from reviewer_validation_failure_history import (
+        assess_validation_failure_history_report as _assess,
+    )
+
+    return _assess(report_text, **kwargs)
+
+
 def assess_infra_stop_handoff_report(report_text, **kwargs):
     """#289: repair handoff purity when infra_stop blocks reviewer capability."""
     from reviewer_infra_stop_handoff import assess_infra_stop_handoff_report as _assess