test(#1514): add fast-check property + all-retired boundary for retired parser

Per review (test-standard items):
- Property test (RULESET.TESTS.property-based-testing): extractRetiredPhaseNumbers
  is the parsing core, so add a fast-check property — k of n checklist phases
  struck → exactly the k canonical keys returned, across randomized phase counts
  and numeric/zero-padded/project-code ID forms. Exposed via a `_`-prefixed test
  seam (mirrors the existing _setLockProbes seams), no public API surface added.
- Boundary (RULESET.TESTS.boundary-coverage): all-retired case (k === n) →
  total_phases 0, via state json.

No production behavior change; the exclusion logic is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/state.cts

@@ -3189,6 +3189,9 @@ export = {
   cmdStateMilestoneSwitch,
   cmdSignalWaiting,
   cmdSignalResume,
+  // Test seam (#1514): the pure retired/folded-phase parser, exposed so its
+  // strikethrough-detection logic can be property-tested directly.
+  _extractRetiredPhaseNumbers: extractRetiredPhaseNumbers,
   // Test seam (audit M1): inject a deterministic isPidAlive so the liveness-gated
   // steal decision is exercised without real pids. Mirrors capability-lock.cts.
   _setLockProbes(probes: Partial<{ isPidAlive: (pid: number) => boolean }>): void {