Thin request-reply layer on channels, no runtime change. A server is an
actor owning a GenServer state; clients hold a clonable ServerRef and issue
call (sync, parks for the reply) or cast (fire-and-forget).
- Single inbox carrying an internal Envelope { Call(req, reply_tx) | Cast },
forced by the no-select / no-unified-mailbox invariant; call makes a
one-shot reply channel and parks on it.
- Server-down detection is pure channel closure (no monitor): send fails if
the inbox is gone; the reply sender drops on the server's unwind so a
parked caller wakes to Err. Both collapse to CallError/CastError::ServerDown.
- init/terminate are optional trait hooks; terminate runs via a drop guard so
it fires on every exit path (clean close, panic, request_stop). Must stay
non-blocking — may run mid-unwind.
- ServerRef carries pid() for monitor/request_stop/link; start + start_under.
Tests: cast-then-call roundtrip, init/terminate ordering, both server-down
paths (reply-channel close on handler panic; inbox-send failure when gone).
