lotus(#6): render avatar decorations on in-call tiles

Adds io.lotus.decorations (toWidget): the host pushes a userId->image-URL
map and EC overlays the profile decoration on each tile avatar
(TileAvatar), keyed by userId, with a useSyncExternalStore-backed store.
Makes A6 first-class in-call instead of absent. URLs are validated
https/blob. Additive: no-op unless the host sends decorations.

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

src/lotus/lotusActions.ts

@@ -25,6 +25,8 @@ export enum LotusWidgetActions {
   InjectAudio = "io.lotus.inject_audio",
   /** toWidget: set audio/screenshare encoding quality (bitrate/framerate). */
   SetQuality = "io.lotus.set_quality",
+  /** toWidget: per-user avatar-decoration image URLs for in-call tiles. */
+  Decorations = "io.lotus.decorations",
 }
 
 /** toWidget Lotus actions that must be allow-listed in `initializeWidget`. */
@@ -32,4 +34,5 @@ export const LOTUS_TO_WIDGET_ACTIONS: LotusWidgetActions[] = [
   LotusWidgetActions.FocusParticipant,
   LotusWidgetActions.InjectAudio,
   LotusWidgetActions.SetQuality,
+  LotusWidgetActions.Decorations,
 ];