lotus(#2): stream per-participant call state over widget API

Opt-in (lotusCallState=1) bridge that emits io.lotus.call_state with each
participant's speaking/audio/video state, so the Lotus host can drive
speaking rings / mute badges / PiP from real events instead of scraping
EC's rendered DOM. Exposes vm.userMedia$ on the public CallViewModel.
Additive: no-op without the flag.

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

src/state/CallViewModel/CallViewModel.ts

@@ -295,6 +295,8 @@ export interface CallViewModel {
    *    multiple devices.
    */
   participantCount$: Behavior<number>;
+  /** [lotus] All participants' user media, exposed for the Lotus call-state widget bridge. */
+  userMedia$: Behavior<WrappedUserMediaViewModel[]>;
   allConnections$: Behavior<ConnectionManagerData>;
   /** Participants sorted by livekit room so they can be used in the audio rendering */
   livekitRoomItems$: Behavior<LivekitRoomItem[]>;
@@ -1721,6 +1723,7 @@ export function createCallViewModel$(
     ),
     allConnections$,
     participantCount$: participantCount$,
+    userMedia$,
     handsRaised$: handsRaised$,
     reactions$: reactions$,
     joinSoundEffect$: joinSoundEffect$,