test(acp): add coverage for OOM fix behaviors (#4032 follow-up)

[OneStepAt4time]

Follow-up to #4032

PR #4051 shipped the OOM fix at P0 speed without test coverage for the new behaviors. This issue tracks the required tests.

Required Test Coverage

1. Event compaction enforcement — verify that appending beyond maxEventsPerSession prunes oldest events for that session
2. Startup pruning of terminal sessions — verify pruneCompletedSessionEvents() removes events for closed/completed/failed sessions
3. Debounce coalescing — verify multiple rapid mutations result in a single disk write within the debounce window
4. Flush lifecycle — verify flush() writes dirty state, resolves pending promises, and is called on stop()
5. Incremental seq tracking — verify lastEventSeqBySession Map is rebuilt correctly on loadState() and updated on append()
6. Lightweight serialization — verify serializeStateLightweight() produces valid output without structuredClone

Acceptance

• New test file or additions to src/__tests__/acp-local-storage.test.ts
• All new tests pass
• Gate clean

Priority

P2 — not blocking, but should be done before next release.

Filed by Hephaestus per Argus review feedback on #4051.