Stack merge hangs after partial merge invalidates latest-push approval

[windsornguyen]

Reproduction

1. Create main <- PR A <- PR B.
2. Enable merge queue and "Require approval of the most recent reviewable push."
3. Approve both PRs after their latest pushes.
4. Merge the stack from PR B.

Expected

Both PRs merge, or the operation stops before either merges.

Actual

PR A merges. GitHub rebases PR B, invalidating its approval. PR B is not queued, and the merge dialog spins indefinitely.

[windsornguyen]

Expanded five-PR reproduction and server-state evidence: #172 (comment)

This reproduction retained APPROVED review decisions after GitHub rewrote the downstream heads, but the remaining queue entries were still removed. The failure is therefore broader than latest-push approval invalidation: the stack landing operation appears to lose downstream queue membership across the squash/restack boundary.