fix(demo/web): memoize metadata track name to avoid re-subscribing per catalog frame

The watch inspector read the advertised metadata track name straight off the
catalog signal, so the subscription effect tore down and recreated the
`meta.json` subscription on every catalog update (e.g. a live encoder-setting
tweak), briefly flickering the metadata panel. Derive the track name through a
memoized computed so the subscription only re-runs when the name or the active
broadcast actually changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BQ9o9paZnRLpYRgbyjUwFV

Author: claude

demo/web/src/index.ts

@@ -381,16 +381,20 @@ ui.run((effect) => {
 // in the catalog's `metadata` list. We read the active broadcast off
 // `broadcast.output.active`, subscribe to that track, and decode the JSON value,
 // re-subscribing whenever the broadcast (or the advertised track) changes.
-ui.run((effect) => {
+// Memoize the advertised track name so the subscription below only re-runs when it
+// (or the active broadcast) changes, not on every catalog frame (e.g. a live
+// encoder-setting tweak rewrites the catalog).
+const metaTrackName = ui.computed((effect) => {
 	const watch = effect.get(activeWatch);
-	if (!watch) {
-		metaSignal.set(undefined);
-		return;
-	}
-
-	const broadcast = effect.get(watch.broadcast.output.active);
+	if (!watch) return undefined;
 	const catalog = effect.get(watch.broadcast.output.catalog) as { metadata?: string[] } | undefined;
-	const trackName = catalog?.metadata?.[0];
+	return catalog?.metadata?.[0];
+});
+
+ui.run((effect) => {
+	const watch = effect.get(activeWatch);
+	const broadcast = watch ? effect.get(watch.broadcast.output.active) : undefined;
+	const trackName = effect.get(metaTrackName);
 	if (!broadcast || !trackName) {
 		metaSignal.set(undefined);
 		return;