fix(review): close the gate's fail-open paths an independent review found

An adversarial read of d88245c reproduced 15 problems; all fixed here, each
with a test that fails on the old code.

- verdict-gate: a greener verdict must be backed by the LAST fence being a
  `json` block of the right shape, with no BLOCKER in the JSON (any case),
  in `## Findings (BLOCKER)`, or a differing finding-id list; two verdict
  lines and more than one file argument are exit 2. ({}, null, lowercase
  "blocker", a trailing ```jsonc and a later json fence all gave exit 0.)
- engine-findings: a transcribed "narrower" key can no longer lower an
  engine tier; absolute paths under --repo-root match the scope; "12" lines
  parse; an empty scope is an error; out-of-scope engine findings are
  unverified (`engine:out-of-scope`) and forbid ALLOW.
- recurrence: the catch-all engine keys and keys normalised from unknown
  ones are never proposed; the same unfixed finding in two iterations is not
  a recurrence; proposals are validated before they are offered; \r/U+2028
  and damaged markers are refused; writes go through a symlinked CLAUDE.md.
  Prose points --repo at the session directory, not the git toplevel.
- Workflow substrate: engines are counted from what arrived, and the
  verdict's floor is recomputed from the coordinator's findings; now tested
  by running the script with stubbed Workflow globals. Two literal NUL bytes
  written as \u0000 (git saw the file as binary).
- Trimming keeps Pass 1's "Also flagged by" provenance; `## Declined to
  Judge` and remediation steps are required from review_version 1.1 only
  (N-1); the zero-diff review now carries the JSON block the gate needs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/lib/coordinator-contract.test.mjs

@@ -157,6 +157,16 @@ test('judgeFilter — only MINOR/SUGGESTION text is trimmed, and every trim is c
   assert.ok(j.kept[0].detail.endsWith('[…trimmed]'), 'the cut is visible in the text');
 });
 
+test('judgeFilter — a trimmed MINOR keeps its "Also flagged by" provenance', () => {
+  const [f] = dedupByTriplet([
+    { file: 'x.mjs', line: 1, rule_key: 'MISSING_ERROR_HANDLING', severity: 'MINOR', title: 'a', detail: 'q'.repeat(900), reviewer: 'code-correctness-reviewer' },
+    { file: 'x.mjs', line: 1, rule_key: 'MISSING_ERROR_HANDLING', severity: 'MINOR', title: 'b', detail: 'r', reviewer: 'claude-code-review' },
+  ]);
+  const j = judgeFilter([f]);
+  assert.ok(j.kept[0].detail.length <= 800);
+  assert.match(j.kept[0].detail, /Also flagged by claude-code-review: b\.$/);
+});
+
 test('judgeFilter — the trim decision reads the catalogue tier, not the reviewer label', () => {
   // MISSING_TEST is MAJOR in the catalogue; a reviewer calling it MINOR must
   // not get its text cut.