lotus(#4): focus-participant spotlight via widget action

Adds io.lotus.focus_participant (toWidget): the host can pin a participant
to the spotlight by Matrix user id (or clear with userId:null), via a
manual override injected into CallViewModel.spotlightSpeaker$. Replaces
cinny's fragile DOM .click() tile-selector focus hack. Extracts the action
enum into lotusActions.ts (no circular import) and allow-lists Lotus
toWidget actions in initializeWidget. Additive: no-op unless the host
sends the action.

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

src/state/CallViewModel/CallViewModel.ts

@@ -255,6 +255,11 @@ export interface CallViewModel {
    * Callback to toggle screen sharing. If null, screen sharing is not possible.
    */
   toggleScreenSharing: (() => void) | null;
+  /**
+   * [lotus] Pin a participant to the spotlight by Matrix user id (#4
+   * focus-participant). Pass null to clear and restore speaker-follows.
+   */
+  setManualSpotlight: (userId: string | null) => void;
   /**
    * Whether we are sharing our screen.
    */
@@ -897,7 +902,7 @@ export function createCallViewModel$(
       merge(userHangup$, widgetHangup$).pipe(map(() => "user" as const)),
     ).pipe(scope.share);
 
-  const spotlightSpeaker$ = scope.behavior<UserMediaViewModel | undefined>(
+  const autoSpotlightSpeaker$ = scope.behavior<UserMediaViewModel | undefined>(
     userMedia$.pipe(
       switchMap((mediaItems) =>
         mediaItems.length === 0
@@ -933,6 +938,29 @@ export function createCallViewModel$(
     ),
   );
 
+  // [lotus] Manual spotlight override (#4 focus-participant widget action).
+  // When set to a userId still present in the call, that participant is
+  // spotlighted instead of the auto-selected active speaker; null restores the
+  // default speaker-follows behaviour. Defaults to null, so without the host
+  // using io.lotus.focus_participant this is a no-op.
+  const manualSpotlightUserId$ = new BehaviorSubject<string | null>(null);
+  const spotlightSpeaker$ = scope.behavior<UserMediaViewModel | undefined>(
+    combineLatest([
+      autoSpotlightSpeaker$,
+      manualSpotlightUserId$,
+      userMedia$,
+    ]).pipe(
+      map(([auto, manualUserId, mediaItems]) => {
+        if (manualUserId !== null) {
+          const pinned = mediaItems.find((m) => m.userId === manualUserId);
+          if (pinned) return pinned;
+        }
+        return auto;
+      }),
+    ),
+    undefined,
+  );
+
   const grid$ = scope.behavior<UserMediaViewModel[]>(
     userMedia$.pipe(
       switchMap((mediaItems) => {
@@ -1699,6 +1727,8 @@ export function createCallViewModel$(
     join: localMembership.requestJoinAndPublish,
     leave: localMembership.requestDisconnect,
     toggleScreenSharing: toggleScreenSharing,
+    setManualSpotlight: (userId: string | null): void =>
+      manualSpotlightUserId$.next(userId),
     sharingScreen$: sharingScreen$,
 
     tapScreen: (): void => screenTap$.next(),