refactor(dev): coalescing rebuild scheduler on Effect fiber + Semaphore + Deferred (wave 3.5 stage 3, PR 3)

[ScriptedAlchemy]

Summary

Subsystem 2 of 4 for #152 stage 3. Reordered ahead of the SSE hub and EpochStore with one sentence of justification: the scheduler is a single self-contained seam inside DevCoordinator with unit-pool-only owning suites, while EpochStore carries cross-process durability contracts — smallest remaining blast radius first. (The SSE hub audit outcome is recorded below.)

• Build passes are Effect fibers. #startBuild runs #performBuild under a Semaphore(1) permit (withPermit); admission already guarantees a single holder, the permit makes the serialization invariant structural rather than implicit.
• Coalescing on Deferred. Every rebuild that arrives while a build runs merges (unchanged mergeInvalidations) into one queued slot whose requesters share a single Deferred<ArtifactEpochResult>; the drain hook (Effect.onExit, sitting exactly where the pre-Effect finally chain sat) starts the follow-up fiber and completes the deferred with its result, failure('DevCoordinator is closing.') on close, or the sentinel failure if the follow-up rejects.
• Admission stays synchronous by design (documented in-code): same-turn rebuilds must observe the running build and coalesce into exactly one follow-up — deferring admission into a fiber would split them.
• Startup cancellation (#awaitStartup races) is deliberately untouched: it is the startup lifecycle, not the rebuild scheduler, and its three blocked-startup suites pin Promise-level sequencing.

SSE hub audit note (subsystem decision, code lands separately)

The planned "PubSub-backed SSE hub" fails the honesty test: the hub's public contract is synchronous re-entrant fan-out — foreground-server.ts documents "EventHub delivery is synchronous" as the invariant that lets shutdown tombstones reach socket buffers before sockets are destroyed, and ~15 tests pin sync replay/removal/ordering. Effect PubSub is an async fan-out structure; fiber delivery changes that observable contract. The hub stays imperative; docs/effect-conventions.md gets the record in the closing docs PR.

Parity (all suites unchanged, zero pin flips)

• Owning suites (dev-coordinator, dev-watcher, dev-server, dev-events, dev-events.contract): 74 passed / 0 failed
• Full unit pool: 1999 passed / 5 documented skips / 0 failed
• pnpm lint + pnpm typecheck: green
• Changeset: patch (internal rewrite).

Refs #152.

[changeset-bot]

🦋 Changeset detected

Latest commit: f939694

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
agent-bundle	Patch

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T17:47:49.410989Z	f939694	PR opened

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