docs(effect): wave 3.5 stage 3 outcomes and helped/hurt addendum

[ScriptedAlchemy]

Summary

Closes out #152 stage 3 in docs/effect-conventions.md:

• Stage-3 outcomes section: per-subsystem records for feat(dev): add the dev-seam Effect boundary module (wave 3.5 stage 3, PR 1) #158/refactor(dev): MCP session lifecycles as Effect scoped resources (wave 3.5 stage 3, PR 2) #159/refactor(dev): coalescing rebuild scheduler on Effect fiber + Semaphore + Deferred (wave 3.5 stage 3, PR 3) #160/refactor(dev): EpochStore staging/leases/recovery on Effect Semaphore + Exit-aware saga (wave 3.5 stage 3, PR 4) #161 and the recorded decision that ProjectEventHub (the SSE hub) stays imperative — its synchronous re-entrant fan-out contract (documented in foreground-server.ts and pinned by ~15 tests) is not what Effect PubSub models.
• Helped/hurt addendum for stage-3 workers and future waves (Semaphore-for-serialQueue, Deferred coalescing, allSettled-as-forEach+Exit, infallible-finalizer consequences, lift.ts rationale, synchronous admission windows).
• Unstable-adoption table: stage 3 adopts none (stable Semaphore/Deferred/Scope/Exit sufficed).

Docs only; no code change.

Refs #152.

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