feat(rivetkit): add Node actor worker threads

[NathanFlurry]

Stacked on #5676 (deferred commits), which is stacked on #5659 (sync SQLite).

• Add experimental actorsPerThread scheduling with CPU-aware placement, fixed per-thread capacity, generation pinning, and idle worker retirement.
• Route actor callbacks directly to worker-local Node-API environments, with worker loss handling and orderly shutdown.
• Fix startup/stop races, generation-scoped callback ownership, bootstrap message handling, retirement cleanup, and bridged error metadata.
• Add worker metrics, public configuration and deployment guidance, and regression coverage for scheduling, teardown, runtime compatibility, and real-worker lifecycle and SQLite behavior.
• Validate on Node 24 with native/wasm builds, TypeScript checks and worker suites, Rust core/NAPI suites, and native driver integration tests.

[railway-app]

🚅 Deployed to the actors-pr-5663 environment in rivet-frontend

Service	Status	Web	Updated
frontend-inspector	😴 Sleeping (View Logs)	Web	Sep 19, 2026 at 3:59 am UTC
frontend-cloud	😴 Sleeping (View Logs)	Web	Sep 19, 2026 at 3:57 am UTC
kitchen-sink	😴 Sleeping (View Logs)	Web	Sep 19, 2026 at 3:55 am UTC
ladle	✅ Success (View Logs)	Web	Sep 19, 2026 at 3:49 am UTC
website	❌ Build Failed (View Logs)	Web	Sep 19, 2026 at 3:46 am UTC
mcp-hub	✅ Success (View Logs)	Web	Sep 4, 2026 at 10:22 am UTC