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.

.fallowrc.jsonc

@@ -34,6 +34,7 @@
     "skills/**/scripts/**",
     "registry/**",
     "examples/**",
+    "playground/**",
     ".github/workflows/fixtures/**",
   ],
   "ignoreExports": [

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;
     }

packages/studio/src/main.tsx

@@ -22,6 +22,16 @@ function errorProps(value: unknown): {
   return { error_message: String(value), error_name: null, stack_trace: null };
 }
 
+function isCompositionAssetError(msg: string): boolean {
+  return msg.includes("Error fetching") && (msg.includes("404") || msg.includes("Not Found"));
+}
+
+const ERROR_CAP = 50;
+let errorCount = 0;
+let rejectionCount = 0;
+let errorCapSent = false;
+let rejectionCapSent = false;
+
 window.addEventListener("error", (event) => {
   if (event.message?.includes("ResizeObserver loop")) {
     event.stopImmediatePropagation();
@@ -29,6 +39,15 @@ window.addEventListener("error", (event) => {
     return;
   }
 
+  errorCount++;
+  if (errorCount > ERROR_CAP) {
+    if (!errorCapSent) {
+      errorCapSent = true;
+      trackStudioEvent("error_cap_reached", { count: errorCount });
+    }
+    return;
+  }
+
   trackStudioEvent("unhandled_error", {
     ...errorProps(event.error),
     error_message: event.message,
@@ -39,7 +58,19 @@ window.addEventListener("error", (event) => {
 });
 
 window.addEventListener("unhandledrejection", (event) => {
-  trackStudioEvent("unhandled_promise_rejection", errorProps(event.reason));
+  const props = errorProps(event.reason);
+  if (isCompositionAssetError(props.error_message)) return;
+
+  rejectionCount++;
+  if (rejectionCount > ERROR_CAP) {
+    if (!rejectionCapSent) {
+      rejectionCapSent = true;
+      trackStudioEvent("rejection_cap_reached", { count: rejectionCount });
+    }
+    return;
+  }
+
+  trackStudioEvent("unhandled_promise_rejection", props);
 });
 
 createRoot(document.getElementById("root")!).render(