acquireWriter

Claim the active-writer slot for writerId. Last-acquirer wins: any actions registered by the previous writer are evicted from the registry. Unbound registrations survive.

Re-acquiring with the same writerId is idempotent — that writer's existing actions are preserved (this matters for attach → onResume → onResume patterns where the same binder may re-acquire without an intervening release).

Use === identity, not equality: binders are typically singleton instances per fragment, and tying GC to identity avoids accidental collisions when two writers happen to be equals-equal.