ChannelRegistry

Ref-counted, thread-aware projection registry.

Owns the spec.key → (store, runtime) mapping for one StreamController. Lifecycle:

• acquire(spec) → +1 ref, returns { store, release }. The first acquire opens the projection's runtime; subsequent acquires for the same key share both the store and the runtime.
• release() → -1 ref. When the last consumer releases, the entry is removed and its runtime disposed.
• bind(thread) → swap or detach the underlying thread; every live entry's runtime is recreated against the new thread, keeping the same store identity.
• dispose() → tear everything down (idempotent). Safe to call multiple times.

The registry is intentionally not generic over a state shape — different consumers can hold projections producing different snapshot types, so the registry keys everything as unknown and lets acquire reapply the caller's T at the boundary.