Reduce checkpoint reactor stream fanout

[juliusmarminge]

Summary

• stop the checkpoint reactor from enqueuing every thread.message-sent event when the pre-turn baseline is already available from thread.turn-start-requested
• keep the server perf harness on the baseline branch and isolate this change as the first stacked optimization
• record the first before/after measurements in performance.md

Perf impact

• create-turn-spam-8x thread.archive dispatch -> thread.archived: 5.71ms / 50.80ms -> 0.93ms / 17.98ms
• create-turn-spam-8x thread.create dispatch -> thread.created: 3.61ms / 58.38ms -> 2.44ms / 67.52ms
• this helped archive latency materially, but did not improve the create tail consistently

Testing

• bun run --cwd apps/server test src/orchestration/Layers/CheckpointReactor.test.ts
• bun run --cwd apps/server test:perf
• bun run fmt .
• bun run lint
• bun run typecheck

Stacked on #1669.

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Note

Medium Risk
Changes the event triggers used to capture pre-turn checkpoint baselines; if thread.turn-start-requested is not emitted in some flows, baseline capture could be skipped and later diff/capture paths may degrade or log warnings.

Overview
Reduces CheckpointReactor domain-event fanout by no longer enqueueing or processing thread.message-sent events, relying solely on thread.turn-start-requested to ensure the pre-turn baseline checkpoint exists.

This narrows ensurePreTurnBaselineFromDomainTurnStart to accept only thread.turn-start-requested and updates the domain event stream filter accordingly, cutting work from the reactor’s hot path.

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Note

Remove thread.message-sent events from CheckpointReactor stream fanout

The CheckpointReactor was routing thread.message-sent events to ensurePreTurnBaselineFromDomainTurnStart, but this path was always short-circuited by an early-return guard. This refactor removes that dead code path entirely, narrowing the event filter and dispatch logic in CheckpointReactor.ts to only handle thread.turn-start-requested, thread.checkpoint-revert-requested, and thread.turn-diff-completed.

^{Macroscope summarized b24b5fb.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR removes 'thread.message-sent' as a trigger for the checkpoint reactor, changing when checkpoints are created. While the author is the primary contributor to this file and the intent is performance optimization, this is a meaningful change to core orchestration behavior that warrants human verification of the intended semantics.

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