fsmonitor: recover status history across mixed writers

[ttaylorr-oai]

Ship the six follow-up fsmonitor/status commits ending at
fca61fdceec3017dff2f5d4b945f6c154290dfdd in the next codex-unstable
release. The reviewed source is a fast-forward merge of the existing
enrolled topic and the exact tested prototype; its tree is byte-identical
to fca61fdcee, and no protected workflow or controller files change.

• Recover authenticated status history after another Git binary rewrites
  the index without FSMN, FSCF, or FSUC. The reported
  git add x; rm x; git checkout -- x; git reset; git status sequence
  remains bounded and reports ? x correctly.
• Revalidate cached directories after fsmonitor provider resets, including
  scoped status requests, without serially reopening the entire worktree.
• Reduce attribute-manifest allocation and parent-revalidation overhead;
  reissue clean proofs when repository fingerprint inputs change.
• Reject unsafe global invalidations, changed attributes, external hardlink
  mutations, and unauthenticated history.

Validation:

• t7527: 156/156 with SHA-1 and SHA-256.
• t7519: 77/77 with SHA-1 and SHA-256.
• t7530: 47/47 with SHA-1 and SHA-256.
• t7063: 64/64 with SHA-1 and SHA-256.
• t7529: 36/36 with SHA-1 and SHA-256.
• 374 unit tests, developer build, and shell test lint.
• Actual OpenAI worktree: root status 524 ms, scoped status 143 ms;
  provider-reset scoped recovery improved from 13.25 s to 5.21 s.

[dreynaud-oai]

Independently reviewed exact head 046008d. Verified its only delta from the previously qualified production tree is one test-only fixture line, and reproduced the formerly failing provider-reset test successfully under SHA-1 and SHA-256. Real dirty macOS and Linux monorepo checkouts show matched-build steady-state performance parity, 836 adversarial macOS checks pass, and all eight Linux fsmonitor watch-failure cases pass.

[dreynaud-oai]

Independently verified exact dad9e3b on the real dirty 858,001-entry Linux monorepo with matching Git client and daemon. New-file add, existing staged-file add, unstage, and dry-run preserve coherent FSMN/FSUC/FSCF and reduce subsequent status from ~19 s to 0.91-1.25 s. Separate live-daemon SHA-1/SHA-256 gates verify unfiltered staging and dry-run preserve history while actual filtered paths fail closed; attribute, text-conversion, and clean-filter safety scenarios also pass.

[dreynaud-oai]

Independently approved exact f92dd57. Real dirty Linux monorepo (858,001 paths, actual system LFS filter): 21 plain/og/preload transitions, 0.95-0.99 s steady status, zero manifest scans and coherent proofs; add/re-add/commit dry-run/unstage remain 0.9-1.23 s. Real dirty macOS monorepo (1,161,336 paths, existing .658 daemon preserved): 21 transitions, 0.25-0.28 s steady status. Active clean-filter fail-closed staging passes 28 scenarios each for SHA-1 and SHA-256. Production source blob independently matched c224fe3876961513c813f18187e3e046fe37ec49; only exact command-scoped acceleration keys are excluded.