Add explicit room leave semantics for future relay connection tasks. Disconnect
handling will need to remove peers from room membership without reaching into the
room internals or leaving stale MAC indexes behind.
Leaving a client now removes both its peer entry and MAC mapping so the same MAC
can rejoin later. Leaving a gateway clears gateway occupancy while preserving any
remaining clients. If the last peer leaves, the room is removed from the
registry. The result reports which peer left and whether the room was removed so
the networking layer can emit lifecycle events cleanly.
Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md relay disconnect reason and reconnect handling groundwork
ddidderr
81ad7abe84
