feat(server): isolate provider commands by thread

[patroza]

Summary

• replace the global provider-command FIFO with lazy FIFO lanes keyed by thread ID
• preserve strict ordering within a thread while allowing unrelated threads to execute concurrently
• reclaim idle lanes immediately and cancel/drop a lane when its thread is deleted, so historical threads allocate no workers
• document the concurrency and lifecycle guarantees

Orchestrator v2 alignment

This matches v2's KeyedSerialExecutor<ThreadId> architecture: serialize within a thread, run unrelated threads concurrently, and remove keyed state after the last user. The v1 worker remains a queue-oriented wrapper because it additionally needs deterministic drain() and explicit cancellation/drop of queued commands on thread deletion.

Tests

• vp check
• ELECTRON_SKIP_BINARY_DOWNLOAD=1 vp run -r --cache --log labeled typecheck
• vp test run packages/shared/src/DrainableWorker.test.ts packages/shared/src/KeyedDrainableWorker.test.ts apps/server/src/orchestration/Layers/ProviderCommandReactor.test.ts apps/server/integration/orchestrationEngine.integration.test.ts
• 58 passed, 1 skipped