fix(706): count a raced SENTINEL, not only a raced human verdict

Round-5 cold review: the post-write check counted only human `Review-verdict:`
rows above the high-water mark, which is not sufficient under the run overlap
this branch measured.

Sequence, all inside that regime, runs A and B on the same exempt-classified
sha: the human BLOCKED lands BELOW A's mark (so A cannot see it), B masks it
with an exemption `success`, and only afterwards writes the sentinel. A then
finds nothing human above its mark, does not repair, and posts its own `success`
on top of the sentinel. The human rejection is permanently green and every later
run re-derives it — the repair race failing toward SUCCESS, while the record
states it fails toward `pending`.

The filter now counts two row shapes above the mark: a human verdict (non-null
creator, `Review-verdict:` description) OR a machine sentinel (null creator,
description exactly $REPAIR_DESC). A then repairs and both runs converge on the
fixed point.

It cannot false-fire: a pre-existing sentinel would have been seen at the FIRST
read and forced the pending path, and this block only runs after a `success`, so
a sentinel above the mark can only have been written mid-flight by another run.

Mutation-verified on both halves independently — dropping the sentinel
alternation reddens the new test; dropping the human half reddens the original
race-2 test — so neither can be removed without a test noticing.

Refs #706

docs/decisions/records/ci/verdict-write-retarget-fence.md

@@ -5,7 +5,7 @@ status: active
 since: '2026-08-03'
 supersedes: none
 superseded-by: none
-rule: 'The `review-verdict/h10` job counts `change_target_branch` events on the PR''s issue timeline at run start and again immediately before its POST, and writes NOTHING if the count moved. The COUNT is the key because the branch NAME is ABA-vulnerable — `main -> S -> main` reads `main` at both ends, which is how #698 route 1 obtained a forged exemption — while the event count is monotonic and cannot alias. Abstaining is a handoff, not a stall, and that is the property the design rests on: every retarget fires `edited`, which is in this workflow''s `types:`, so the event that makes a run abstain has already queued a successor whose window opens after it; the induction terminates when retargeting stops and the last run writes the final answer. `updated_at` was REJECTED as the key because it also moves for comments and labels, which fire none of this workflow''s `types:` — a run could abstain with no successor coming, which is a real stall. The count is trusted only when paging reached a validated EMPTY page; an untrusted count (unreadable page, non-array body, non-numeric length, page cap hit) blocks the exemption `success` ONLY and still lets `pending` through, because `pending` cannot turn an unreviewed head green while withholding it would strand ordinary PRs for no safety gain. SEPARATELY, and for the human-verdict race the fence does nothing about: after posting an exemption `success` the job re-reads `/statuses/{sha}` and, if a human `Review-verdict:` row appeared with an id ABOVE a high-water mark taken just before the POST, overwrites its own status with `pending` and logs an error. The repair is `pending`, NEVER a copy of the human''s state, since re-posting their `failure` under the machine credential would attribute a human verdict to the job; its description is a SENTINEL that the classification refuses to grant an exemption over AND re-writes verbatim on every later run, so the block is a FIXED POINT rather than decaying — writing the generic `pending` description there instead erases the marker and the exemption simply returns one event later. The mark is captured BEFORE the last-moment re-read, not merely before the POST — a later mark leaves a multi-round-trip blind gap in which a verdict is neither seen by the re-read nor repaired afterwards. The id comparison is load-bearing: a mere presence test would fire forever on a base-mismatched verdict that `read_existing_verdict` deliberately declines to honour, deadlocking that PR''s exemption permanently. Finally, a run whose last-moment re-read finds a sentinel it did not see at its FIRST read ABSTAINS instead of posting: that can only mean an overlapping run repaired a raced verdict mid-flight, and this run''s `success` — frozen at classification time, with the human row below its own mark, so neither the fence nor the post-write check would catch it — would otherwise bury the rejection. That is the one path in this design that failed toward SUCCESS rather than `pending`.'
+rule: 'The `review-verdict/h10` job counts `change_target_branch` events on the PR''s issue timeline at run start and again immediately before its POST, and writes NOTHING if the count moved. The COUNT is the key because the branch NAME is ABA-vulnerable — `main -> S -> main` reads `main` at both ends, which is how #698 route 1 obtained a forged exemption — while the event count is monotonic and cannot alias. Abstaining is a handoff, not a stall, and that is the property the design rests on: every retarget fires `edited`, which is in this workflow''s `types:`, so the event that makes a run abstain has already queued a successor whose window opens after it; the induction terminates when retargeting stops and the last run writes the final answer. `updated_at` was REJECTED as the key because it also moves for comments and labels, which fire none of this workflow''s `types:` — a run could abstain with no successor coming, which is a real stall. The count is trusted only when paging reached a validated EMPTY page; an untrusted count (unreadable page, non-array body, non-numeric length, page cap hit) blocks the exemption `success` ONLY and still lets `pending` through, because `pending` cannot turn an unreviewed head green while withholding it would strand ordinary PRs for no safety gain. SEPARATELY, and for the human-verdict race the fence does nothing about: after posting an exemption `success` the job re-reads `/statuses/{sha}` and, if a human `Review-verdict:` row appeared with an id ABOVE a high-water mark taken just before the POST, overwrites its own status with `pending` and logs an error. The repair is `pending`, NEVER a copy of the human''s state, since re-posting their `failure` under the machine credential would attribute a human verdict to the job; its description is a SENTINEL that the classification refuses to grant an exemption over AND re-writes verbatim on every later run, so the block is a FIXED POINT rather than decaying — writing the generic `pending` description there instead erases the marker and the exemption simply returns one event later. The mark is captured BEFORE the last-moment re-read, not merely before the POST — a later mark leaves a multi-round-trip blind gap in which a verdict is neither seen by the re-read nor repaired afterwards. The id comparison is load-bearing: a mere presence test would fire forever on a base-mismatched verdict that `read_existing_verdict` deliberately declines to honour, deadlocking that PR''s exemption permanently. Finally, a run whose last-moment re-read finds a sentinel it did not see at its FIRST read ABSTAINS instead of posting: that can only mean an overlapping run repaired a raced verdict mid-flight, and this run''s `success` — frozen at classification time, with the human row below its own mark, so neither the fence nor the post-write check would catch it — would otherwise bury the rejection. That is the one path in this design that failed toward SUCCESS rather than `pending`. The post-write check counts TWO row shapes above the mark, not one — a human `Review-verdict:` row AND a machine sentinel — because with two overlapping runs the human row can sit BELOW the second run''s mark while the first masks it and only then writes the sentinel, leaving the second to post its own `success` on top; counting the sentinel converges both runs on the fixed point instead.'
 signals: 'stale review-verdict run overwrites a fresher one, retarget ABA against the docs-only classifier, concurrency group does not serialize pull_request_target, gitea auto-cancel push vs pull_request_target, forged exemption restored after reclassification, human BLOCKED silently turned green, post-write status verification, change_target_branch timeline count, why does my PR post no verdict status after a retarget · paths: `.gitea/workflows/review-verdict.yml`, `scripts/tests/test_pr_changed_files.py` · issues: #706, #698, #672, #663, #622'
 mechanics: '`count_retargets()` pages `GET /repos/{repo}/issues/{pr}/timeline?limit=50&page=N` (cap 20) setting `rt_count`/`rt_ok`, trusted only on a validated empty page; `retargets_before`/`retargets_before_ok` captured before enumeration, re-counted immediately before the POST; `max_id_before` from `GET /repos/{repo}/statuses/{sha}` (a BARE ARRAY, unlike the combined `/commits/{sha}/status` object); repair POST is `pending`; tests `test_a_RETARGET_DURING_the_run_posts_NOTHING`, `test_a_PR_retargeted_BEFORE_the_run_but_QUIET_during_it_is_STILL_exempt`, `test_an_UNTRUSTED_retarget_count_withholds_the_EXEMPTION`, `test_an_UNTRUSTED_retarget_count_STILL_LETS_PENDING_THROUGH`, `test_a_human_verdict_landing_AFTER_the_POST_is_repaired_to_pending`, `test_a_PRE_EXISTING_human_row_does_NOT_trigger_a_repair`'
 ---