On a long subagent-heavy run the TUI thought stream froze for 1.5–4 s
at a time, with the loop watchdog blaming `subagent:*` phases and the
log carrying ~1,255 `Skipping mid-run compaction because turn persistence
is out of order` debug lines per session (issue #3629).
`#persistTurnMessagesForMidRunCompaction` and the persistence-check
helpers it calls were doing two O(n²) things per `onTurnEnd`:
1. Every call rebuilt the branch path via `SessionManager.getBranch()`
(O(branchSize) `unshift` per call), once per turn message twice (once
in `#sessionMessageAlreadyPersisted`, once in
`#hasPersistedLaterTurnMessage`).
2. Every pairwise message comparison serialized full message content
through `#messageValueSignature` (=`JSON.stringify`), even though
the content was almost always orthogonal to the identity decision.
Replace the structural comparator with a stable persistence key
(timestamp + role-specific discriminators) extracted to a sibling module
`session/turn-persistence.ts` and drive the planner off one snapshot of
the branch built per `onTurnEnd`. Content equality is retained as the
slow-path tiebreaker for the rare case where two messages collide on the
cheap key (e.g. two assistant turns at the same millisecond with
`undefined` responseId — the shape the test harness emits).
The new helpers (`sessionMessagePersistenceKey`, `planTurnPersistence`,
`sameMessageContent`) are pure functions covered by direct unit tests
in `test/turn-persistence.test.ts`; the existing mid-run / eager
compaction integration tests prove the end-to-end behavior is preserved.
Fixes#3629
roboomp