fix(history): serialize collision rebuilds with appends

[Duang777]

Summary

• take the per-goal run-index lock for the complete collision rebuild transaction
• serialize snapshot reads, recovery artifact creation, backup, and index replacement with normal append writers
• add a deterministic concurrent regression test that pauses before replacement and proves a new append survives

Root cause

apply_reviewed_collision_rebuild rewrote index.jsonl from an unlocked snapshot. A normal run append could commit after that snapshot and then be overwritten by the rebuild replacement.

Validation

• python -m pytest -q tests/test_history_index_write_serialization.py tests/test_history_chronology.py tests/control_plane/test_run_artifact_reservation.py tests/control_plane/test_quota_void_commit_runtime.py (33 passed)
• python examples/history-index-duplicate-inspection-smoke.py
• python -m mypy loopx/control_plane/runtime/run_index_rebuild.py
• python -m ruff check loopx/control_plane/runtime/run_index_rebuild.py tests/test_history_index_write_serialization.py
• python -m loopx.cli canary premerge --from-git-diff --tier standard --format json --no-progress (passed, 13 selected checks)
• python examples/docs-governance-smoke.py

Known baseline failure

python examples/repository-hygiene-smoke.py reports the existing missing release timeline entry for v1.2.0; the same failure reproduces on unchanged origin/main.

Merge policy

Control-plane behavior change. Maintainer review and merge required; do not self-merge.

[Duang777]

Exact-head self-review for 67bc85d: APPROVE for maintainer review. The rebuild now holds the shared run-index lock across collision detection and replacement, and the regression test exercises a concurrent append during rebuild. All branch-specific CI checks passed. The remaining installed-artifact failure is the main-branch packaging defect tracked by #5096.

[Duang777]

Exact-head self-review for f9e43f87735ae373aaa7e22e884efba4e68143cb: ready for maintainer review. This head merges origin/main@eaa0c0fd089e5844b763cc24132dcf8a3881c39b without conflicts and retains only the run-index serialization fix plus its regression test. Local validation passed: 33 focused Python tests, Ruff, Mypy, history-index smoke, docs governance, and the standard premerge gate with 13 checks.

[Duang777]

CI triage for exact head f9e43f87735ae373aaa7e22e884efba4e68143cb: all non-Python-shard checks passed, including PostgreSQL, release build, TypeScript, Windows, stage2c, and DCO. The four Python shards fail on the timeout-aware authority calls merged by #5063: existing test doubles reject the new timeout keyword, and the registry I/O manifest is stale. This branch changes only run-index rebuild code and its regression test; the same failures reproduce on current main. The isolated baseline repair is #5104. I am leaving this branch unchanged until that fix lands, then I will merge current main and rerun exact-head CI.