Split soak-stabilization PR into reviewable fixes

## Summary

PR #1265 has become a validated integration branch for the soak-stabilization work, but it is too large to review and merge directly. Split it into focused, reviewable PRs from `origin/main`.

## Context

- Integration PR: #1265
- Integration branch: `fix-soak-sync-checkpoint-stability`
- Validated head: `51dda8e9301a324a27bad9ac27c332d46c7f790f`
- Live soak check on 2026-05-15 showed PR-1265 workers running for roughly 27 hours with no active failure clusters and zero recent failed verifications in the incident windows.

## Reviewable Work

- [x] #1274
- [x] #1275
- [x] #1276
- [x] #1277
- [x] #1278
- [x] #1279
- [x] #1280
- [x] #1281

## Working Rules

- Start each child PR from `origin/main`, not from the full integration branch.
- Keep each PR focused on one production concern with its own tests.
- Reference #1265 and the validated head SHA in each child PR as soak evidence.
- Do not replay the investigation history one commit at a time; fold intermediate experiments and rollbacks into coherent final changes or drop them.
- Keep #1265 as the reference branch until the split PRs have landed or been intentionally superseded.

## Related

- Supersedes direct review of #1265 as a merge target.
- Related to #1221 and #1228.

Start with integration PR #1265, branch fix-soak-sync-checkpoint-stability, and validated head 51dda8e9301a324a27bad9ac27c332d46c7f790f. Review child PRs #1274 through #1281 to verify they start from origin/main, remain focused, include their own tests, and cite the soak evidence. The split is complete when the child PRs have landed or been intentionally superseded while #1265 remains the reference branch.

Written by the indexing model from the issue text.