fix(signal): add call_id to Hangup — prevents stale hangup killing new calls

Root cause: Hangup had no call_id field. The relay forwarded hangups to
ALL active calls for a user. When user A hung up call 1 and user B
immediately placed call 2, the relay's processing of A's hangup would
also kill call 2 (race window ~1-2s).

Fix: add optional call_id to Hangup (backwards-compatible via serde
skip_serializing_if). When present, the relay only ends the named call.
Old clients send call_id=None and get the legacy broadcast behavior.

Also: clear pending_path_report in Hangup recv handler and
internal_deregister to prevent stale oneshot channels from blocking
subsequent call setups.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-client/src/featherchat.rs

@@ -170,6 +170,7 @@ mod tests {
 
         let hangup = SignalMessage::Hangup {
             reason: wzp_proto::HangupReason::Normal,
+            call_id: None,
         };
         assert!(matches!(signal_to_call_type(&hangup), CallSignalType::Hangup));