Parser fuzz validation targets: findings ledger and kill criterion (#1781 B2)

[thymikee]

The parser fuzz lane's validation targets (cli-validation, maestro-validation, added in #1866 for #1781 B2) ship with a kill criterion that nothing currently records, so this is their live tracker — the same treatment #1823 gave the subprocess-stub project.

What the lane costs

Nightly Parser Fuzz Lane in .github/workflows/replays-nightly.yml: 38,000 cases x 7 targets, measured at parity with the pre-B2 five-target/50k budget (16.9s vs 16.8s on a quiet host; the CI step was 21s before). No new job, no macOS occupancy. PR-time cost is scripts/fuzz/validation-arbitraries.test.ts (~0.4s, unit-core) plus the corpus replay that already existed.

Findings ledger (append one row per nightly finding)

date	run	target · class	real defect or phantom?	outcome
(none yet)

A phantom is a case whose expectation was wrong rather than the parser — a generator drift, not a bug. One was caught pre-merge (--scale=1.110000000000017, float modulo drifting past a fractional max) and fixed before landing; validation-arbitraries.test.ts exists to catch that class at PR time.

Kill criteria (either fires ⇒ delete the two validation targets, keep the classic five)

1. Phantoms: a phantom finding reaches a nightly twice.
2. Yield: no real defect found by either validation target in 6 months of nightlies — review on 2027-02-19.

The calibration behind the lane (#1781, B3 comment) measured reach, not yield: it proved the targets can rediscover seeded defects of the shape they aim at, and explicitly did not predict how many unknown defects they will find. Criterion 2 is what tests yield, which is why this ledger exists.

Notes for whoever reviews this

• Only two of the eight calibration rows were genuine generated new reach (fix(cli): reject excess positionals #1433 via cli-validation; a silently-accepted Maestro field via maestro-validation); two more were new detection class but rediscovered by pinned seed cases, and four were already within the classic targets' reach. Judge yield against the first two.
• Adding a mutation class is a few lines in scripts/fuzz/validation-arbitraries.ts; classes whose whole input space is a handful of strings belong in the target's seed list instead.

Umbrella: #1781 (item B2/B3). Lane: #1414. Sibling tracker: #1823.