[core] Settle a hook's awaiter in-process instead of re-invoking, on creation and on conflict (#3938)

* [core] Settle a hook's awaiter in-process instead of re-invoking, on creation and on conflict

* [core] Address review: deterministic hook signal tests, split changesets, document the boundary

- hook.test.ts: drive the idle poll with explicit macrotask turns instead of a
  fixed 20ms sleep (Copilot)
- Split the changeset so each package's entry says only what changed in it
- runtime-tuning docs: hook-only suspensions no longer always park; the hook
  write continuation is the one exception

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>