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/tile/TileAvatar.module.css

@@ -18,3 +18,20 @@ Please see LICENSE in the repository root for full details.
   /* TODO: make this --cpd-color-fg-primary when available. */
   color: var(--cpd-color-text-primary);
 }
+
+/* [lotus #6] Wrap so the decoration can overlay the avatar precisely. */
+.container {
+  position: relative;
+  display: inline-flex;
+}
+
+/* [lotus #6] APNG/PNG profile decoration drawn over the in-call tile avatar.
+The decoration art shares the avatar's box (transparent padding, ring at the
+edges), matching how the host renders it elsewhere. */
+.lotusDecoration {
+  position: absolute;
+  inset: 0;
+  margin: auto;
+  pointer-events: none;
+  object-fit: contain;
+}