Add unit tests for useOrchestratorState buffered flush mechanism

## Context

The `useOrchestratorState` hook uses a rAF-based buffered flush mechanism to batch rapid-fire orchestrator events into single React state updates. A race condition was discovered and fixed where post-flush event handlers would fall back to a stale `initial` closure, reverting already-loaded PVM states to pre-`loadProgram` defaults. This caused false divergence when enabling/disabling the second PVM.

The fix (adding a `lastFlushedSnapshots` ref) is in place, but the existing E2E tests cannot reliably trigger the race condition since both web workers typically respond within the same animation frame.

## Proposed work

Add unit tests for `useOrchestratorState` that mock `requestAnimationFrame` and orchestrator events to deterministically test:

1. **Post-flush event preservation** — after PVM A's `pvmStateChanged` is flushed via rAF, PVM B's event should build on A's flushed state, not the stale `initial`
2. **Multiple flush cycles** — accumulated state survives across multiple rAF flush boundaries
3. **Orchestrator switch cleanup** — when the orchestrator changes, `lastFlushedSnapshots` and `pendingSnapshots` are properly reset, preventing cross-orchestrator state leakage
4. **Concurrent PVM loading** — simulating `loadProgram` where workers respond at different times across rAF boundaries

## Technical notes

- Vitest's `vi.useFakeTimers()` doesn't mock rAF by default; may need explicit `vi.stubGlobal('requestAnimationFrame', ...)` or `--environment jsdom`
- The hook depends on `OrchestratorContext` — tests will need a mock orchestrator (EventEmitter implementing `OrchestratorEvents`) wrapped in the provider
- Pre-existing issue: many unit tests fail with `document is not defined` due to missing jsdom environment config — fixing that would be a prerequisite

## Related

- Fix commit on branch `td-fix-pvm-disable-diverge`
- File: `apps/web/src/hooks/useOrchestratorState.ts`

Start with apps/web/src/hooks/useOrchestratorState.ts and inspect its OrchestratorContext usage, pendingSnapshots, and lastFlushedSnapshots handling. Check the unit-test setup and resolve the noted document is not defined/jsdom prerequisite before mocking requestAnimationFrame and an EventEmitter-style orchestrator. Done means deterministic tests cover post-flush preservation, multiple flush cycles, orchestrator switching, and staggered PVM loading.

Written by the indexing model from the issue text.