fix(studio): stop composition fetch-404 flood and cap error telemetry

Two fixes for the 3M+ unhandled_promise_rejection events/day spike:

1. Filter: suppress "Error fetching ... 404" rejections from composition
   code — these are asset-not-found content errors, not Studio bugs.

2. Rate-limit: cap both error and rejection telemetry at 50 per session.
   After the cap, emit a single *_cap_reached event so we know capping
   occurred without generating unlimited events.

3. Root cause: webAudioTransport now checks response.ok before decode
   and caches failed URLs in _failedSrcs so repeat ticks don't re-fetch
   the same 404 on every playback frame.

Also add playground/ to fallow ignorePatterns — local experiment
directory was tripping the audit gate.

packages/core/src/runtime/webAudioTransport.ts

@@ -18,6 +18,7 @@ export type ScheduledSource = {
 export class WebAudioTransport {
   private _ctx: AudioContext | null = null;
   private _bufferCache = new Map<string, AudioBuffer>();
+  private _failedSrcs = new Set<string>();
   private _activeSources: ScheduledSource[] = [];
   private _masterGain: GainNode | null = null;
   // Composition-time reference frame: at AudioContext time `_rateAnchorCtx`,
@@ -53,14 +54,21 @@ export class WebAudioTransport {
     const src = el.currentSrc || el.getAttribute("src");
     if (!src) return null;
     if (this._bufferCache.has(src)) return this._bufferCache.get(src)!;
+    if (this._failedSrcs.has(src)) return null;
     if (!this._ctx) return null;
     try {
       const response = await fetch(src);
+      if (!response.ok) {
+        this._failedSrcs.add(src);
+        swallow("webAudioTransport.fetch", new Error(`${response.status} ${src}`));
+        return null;
+      }
       const arrayBuffer = await response.arrayBuffer();
       const audioBuffer = await this._ctx.decodeAudioData(arrayBuffer);
       this._bufferCache.set(src, audioBuffer);
       return audioBuffer;
     } catch (err) {
+      this._failedSrcs.add(src);
       swallow("webAudioTransport.decode", err);
       return null;
     }