feat(TASK-020): finding-verifier + step-runner fail-closed hardening

finding-verifier: parse gains max_attempts 2 + parse_failure capture +
fallback to a new parse_fault marker that rewrites verdicts to a single
UNVERIFIABLE PIPELINE-FAULT entry and continues to done (sentinel
preserved — the caller's verify lane degrades instead of dying);
verify_one gains a branch-local verify_fault marker normalizing the
engine failure text into a schema-conformant UNVERIFIABLE verdict.

step-runner: implement gains fallback end_failure (no retry — reruns
against a mutated tree are unsafe; STEP_FAILED renders the failure via
coder_result); independent_review gains fallback write_handoff with the
handoff instructions and route_review treating an "Agent node failed:"
review_report as review-did-not-run (fail-visible, guard ahead of the
🔴 grep); orient/edge_case_sweep/write_handoff capture their failure
text via state_updates, shrinking the bundled fallback-capture warning
baseline back to zero.

R5 tests: fault-marker normalization + crash paths (raw non-JSON
GRAPH_STATE), route_review.sh fault/critical/clean routing via bash+jq,
and structural wiring tests pinning fallbacks, sentinels, and
end_failure's initial_state-backed template keys.

assets/agents/step-runner/scripts/route_review.sh

@@ -13,6 +13,16 @@ review_report=$(echo "$state" | jq -r '.review_report // ""')
 review_attempts=$(echo "$state" | jq -r '.review_attempts // 0')
 max_review_attempts=$(echo "$state" | jq -r '.max_review_attempts // 1')
 
+# Fail-visible guard: a review_report holding the engine's "Agent node
+# failed:" text means the independent review DID NOT RUN (its fallback
+# normally routes straight to write_handoff, bypassing this script). Never
+# treat that text as findings or spend a fix-loop attempt on it -
+# write_handoff flags the fault prominently in the handoff instead.
+if [[ "$review_report" == "Agent node failed:"* ]]; then
+  jq -nc '{"_next": "write_handoff"}'
+  exit 0
+fi
+
 if ! grep -qF "🔴" <<< "$review_report"; then
   jq -nc '{"_next": "write_handoff"}'
   exit 0