feat(tui): Phase 1 — Observer foundation, scheduler slots, history

[Exelord]

Summary

Phase 1 of the TUI roadmap (specced in PR #72): orchestrator-side
scaffolding. No renderer in this PR — the surface is
renderer-agnostic so the same Observer can feed a future vx ui
historical browser and embedder use cases. Phase 2 (OpenTUI
components + --tui flag) builds on top.

No behavior change for non-TUI runs. Tests: 414 → 434.

What ships

1. src/orchestrator/observer.ts — tagged-union ObserverEvent
   (runStart | taskStart | taskStdout | taskStderr |
   cacheProbe | taskComplete | remoteCache | runEnd), an
   Observer interface, and makeSafeObserver(inner) that swallows
   throws from inner.emit so a buggy consumer never crashes the run.
   Logger stays parallel — it owns terminal framed-block output;
   Observer is the structural sink.

2. RunOptions.observer wired through orchestrator.run().
   Emit sites match the design doc (docs/design/tui-design.md §3):

   • runStart after header writes.
   • taskStart from the scheduler's onStart(node, slot).
   • cacheProbe from execute-task.ts after cache.get(hash)
     (hit-local | hit-remote | miss | no-cache).
   • taskComplete from scheduler's onFinish.
   • runEnd after formatRunSummary, before --summarize /
     --profile writes.

3. Scheduler worker-slot allocation. runGraph now allocates
   lowest-free-index slots from [0, concurrency) and passes
   slot: number to execute() + onStart(). Stable allocation so
   a future Workers view renders [1] always-busy / [N] idle-gap
   visibly across runs.

4. Cache.getTaskHistory(taskIds) — one SQL CTE with
   ROW_NUMBER() OVER (PARTITION BY project, task) capped at 50 rows
   per pair. Returns TaskHistoryMap of runs / avg / p50 / p99 / successRate / hitRate / recent[10]. Threaded through prepareRun
   so the runStart event carries historyTable populated for every
   node in the graph (cheap; one batched read).

5. LayeredCacheOptions.onRemoteRequest callback fires for every
   remote GET/PUT with { op, hash, bytes?, latencyMs, ok }.
   wrapWithRemoteCache(local, log, observer) forwards it into
   observer.emit({ kind: 'remoteCache', ... }).

Test plan

• bun src/bin.ts run format — clean
• bun src/bin.ts run lint — 0 warnings, 0 errors
• bun src/bin.ts run test — 434 pass, 0 fail
• New: 5 observer (tests/observer.test.ts), 5 scheduler-slot
  (tests/scheduler.test.ts), 4 cache-history
  (tests/cache.test.ts), 3 layered-cache callback
  (tests/layered-cache.test.ts), 3 end-to-end observer wiring
  (tests/orchestrator.test.ts)
• Existing 414 still green (unchanged behavior)

https://claude.ai/code/session_016HXj6HW6bxSn8EYuKcxTD9

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