feat(cache-warming): CR-3 — manual warm resets timer + nextWarmAt/lastWarmAt surface

FE CR-3 (backend-handoff-cache-warming-timer.md). The inversion: session-orchestrator's
warm() (the single chokepoint for manual /chat/warm AND the automatic timer) emits a
warmCompleted bus event; cache-warming subscribes and does ALL post-warm handling. So a
manual warm now re-arms the timer + refreshes the surface with NO transport-http change
(core can't depend on the standard cache-warming ext).
- session-orchestrator: warmCompleted event hook + emit from warm() on success
- cache-warming: warmCompleted subscriber unifies result handling (manual + automatic);
  adds nextWarmAt/lastWarmAt state + a custom 'cache-warming-timer' surface field
- fix: createWarmService was missing the emit dep (deps.emit?. silently no-oped) →
  wired it + made emit REQUIRED so it can't regress

Live-verified vs claude haiku: manual POST /chat/warm now logs cache-warming 'warm
complete' ~2s after the turn (not the 4-min timer) → manual warm reaches the warmer.
800 vitest + 109 bun green; tsc -b 0; biome clean.

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