fix(protocol): deliver self-initiated state changes to the actor too

A connected Windows client would randomly snap from its joined channel
back to Lobby. Root cause was a state-sync inconsistency, not a drop:
the server delivered self-initiated state changes (channel join/leave,
stream announce/stop) only as a private *Result to the actor and
broadcast the authoritative UserEvent::UPDATED to everyone else. The
core never applied the result to its SessionModel, so vc_list_users()
kept self in the old channel; the Windows HandleUserUpdated rebuilds
_currentChannelId from vc_list_users() on any user's UPDATED event, so
the next unrelated event surfaced the stale self-channel.

Fix, per the response-vs-broadcast contract now documented in
docs/protocol.md §6: the *Result is pure ack/correlation/actor-private
payload; the resulting state change is broadcast to every client
INCLUDING the actor, and clients apply it to their local model rather
than re-deriving own state from a *Result.

- server: join/leave/stream announce+stop broadcast with exclude=0
- server: text fan-out includes the sender (channel + private echo)
- core: response handlers no longer mutate session_model_
- windows: drop optimistic text echo; render own message via the relay
- docs/protocol.md §6: document the response-vs-broadcast contract

Registry-level admin broadcasts (move/mute/kick/channel CRUD) already
used exclude=0 and were correct. ctest build/m1-dev 18/18 green;
VoiceCat.App builds 0 warnings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/src/core/client.cpp

@@ -532,6 +532,9 @@ void vc_client::handle_channel_event(const voicecat::v1::ChannelEvent& ce) {
 }
 
 void vc_client::handle_join_channel_result(const voicecat::v1::JoinChannelResult& msg) {
+    // This response only acks the request + carries our private audio config. The
+    // authoritative channel change reaches us as a UserEvent::UPDATED broadcast (the server
+    // no longer excludes the mover), which keeps session_model_ in sync via apply_user_event.
     vc_event ev{};
     ev.type       = VC_EVENT_JOIN_RESULT;
     ev.result     = msg.ok() ? VC_OK : VC_ERR_PROTOCOL;