Type-safe WebSocket handler registration as a first-class concept rather than relying on the `Raw` escape hatch.
Scope:
- `WebSocket(reg, pattern, handler)` registration function
- Typed message handler signature
- OpenAPI spec: document WebSocket endpoints
- Connection lifecycle hooks (open, close, error)
Type-safe WebSocket handler registration as a first-class concept rather than relying on the `Raw` escape hatch.
Scope: