feat(eval): SKAL-1·4b offline gold-scored output eval

Scores committed agent-run fixtures against the golden corpus at
(file, rule_key) granularity, building on the deterministic coordinator
contract (4a). Offline: committed reviewer payloads, no live agent spawn,
no LLM, no network (the LLM-in-the-loop grading is the separate 4c tier).

- lib/review/gold-scorer.mjs: scoreFindings (precision/recall/f1 at
  (file,rule_key) granularity, line+severity ignored) + scoreVerdict; pure,
  with documented vacuous-set conventions.
- tests/fixtures/bakeoff-rich/runs/run-perfect.json: committed run that
  reproduces all 5 seeded gold findings through runContract.
- tests/lib/gold-eval.test.mjs: the scoring RUN (precision/recall/f1 = 1.0,
  verdict == expected_verdict BLOCK, nothing suppressed/skipped).
- lib/util/test-census.mjs: third census category (goldEval) — a scoring run
  is neither behavior coverage nor a doc-pin; honest-count invariant now 3-way.
- docs/eval-corpus/README.md: 4b moved from Future hardening to implemented.

Suite 809 -> 822 (820/0/2). gold-scorer covers TP+FP+FN+degenerate paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BJQYC5vpkJWxndS55vQQZ6

docs/eval-corpus/README.md

@@ -7,9 +7,11 @@ review/coordinator agent **misfires** on a real task, the case is distilled into
 a machine-readable record and added here, so the failure can never silently
 regress.
 
-This tier is the **staging ground** for the offline gold-scored output eval
-(SKAL-1·4b). It is deliberately **not wired into CI** yet — these are committed
-fixtures + a schema, not a scoring run.
+This corpus is the gold that the **offline gold-scored output eval (SKAL-1·4b)**
+scores against — that eval is now implemented and wired into `node --test`
+(see [§Gold-scored output eval](#gold-scored-output-eval-skal-14b) below). The
+corpus records here are committed fixtures + a schema; the scoring run is the
+separate piece that consumes them.
 
 ## Seed example
 
@@ -65,10 +67,38 @@ A corpus file is one JSON object:
 3. Add (or extend) a loader test in the `tests/lib/gold-corpus.test.mjs` shape so
    the record's shape + catalogue membership are pinned under `node --test`.
 
+## Gold-scored output eval (SKAL-1·4b)
+
+The offline scoring run that grades a recorded agent run against this corpus.
+**Offline** = committed reviewer payloads, no live agent spawn, no LLM, no
+network (the LLM-in-the-loop grading is the separate 4c tier).
+
+- **Committed runs** live under `tests/fixtures/bakeoff-rich/runs/`. Each file is
+  the JSON **reviewer payloads** (one object per reviewer, `{ reviewer, findings }`)
+  that a recorded run produced. `run-perfect.json` is the regression guard: fed
+  through the coordinator contract it must reproduce every seeded gold finding.
+- **The contract** `lib/review/coordinator-contract.mjs::runContract(payloads)`
+  turns those payloads into the deterministic coordinator output (4a).
+- **The scorer** `lib/review/gold-scorer.mjs`:
+  - `scoreFindings(runFindings, goldFindings)` matches at **`(file, rule_key)`
+    granularity** (line + severity ignored) → `{ tp, fp, fn, precision, recall,
+    f1, matched, missed, spurious }`. Vacuous-set conventions (empty run →
+    recall 0; empty gold → precision 0; f1 collapses to 0) are documented in the
+    module header.
+  - `scoreVerdict(runVerdict, goldVerdict)` → exact verdict match.
+- **The scoring run** `tests/lib/gold-eval.test.mjs` asserts `run-perfect`
+  reproduces gold at precision/recall/f1 = 1.0 and `verdict === expected_verdict`.
+- **Third test-census category.** `lib/util/test-census.mjs` now reports a
+  `goldEval` bucket (matched by `GOLD_EVAL_FILE_RE`) separately from `behavior`
+  and `docPins` — a scoring run is neither behavior coverage nor a prose pin, so
+  the honest-count invariant is now a 3-way sum.
+
 ## Future hardening (not in this tier)
 
-- SKAL-1·4b: an offline gold-scored output eval that scores recorded agent runs
-  against these records at `(file, rule_key)` granularity, with committed runs
-  and a third test-census category.
 - A prose↔JSON cross-assertion (parse the fixture README table, diff against
   `gold.json`) to mechanically bound the two-source-of-truth drift.
+- Degraded-run fixtures (a run that misses or invents findings) to exercise the
+  scorer's discriminating path end-to-end; today that path is covered by
+  `tests/lib/gold-scorer.test.mjs` with inline synthetic findings.
+- SKAL-1·4c: the LLM-in-the-loop eval that grades live agent runs (needs a
+  filesystem + model judgement, deliberately excluded from this deterministic tier).