fix(studio): enable timeline resize for all elements, improve perf and UX

Enable trim-start and trim-end for all authored timeline elements (divs,
sections, compositions) — not just video/audio/img. The deterministic-window
gate was overly restrictive since all non-implicit elements have authored
data-start/data-duration that define their timeline window.

Replace iframe reload after resize/move with direct DOM attribute patching
via patchIframeDomTiming(). This eliminates playhead-jump-to-zero, visual
blinking, and race conditions from file-watcher echoes. File persistence
runs in a serialized background queue (persistTimelineEdit + enqueueEdit)
so rapid edits don't overwrite each other.

Add mediabunny-based media probe service (mediaProbe.ts) for fast metadata
extraction from file headers. Timeline elements missing sourceDuration are
enriched asynchronously without waiting for DOM loadedmetadata events.

Tune the runtime media preloader: lower lazy threshold from 6 to 3 clips,
add 3s lookbehind window for reverse scrub, adaptive promoted-clip cap.

Deduplicate getTimelineEditCapabilities — computed once in TimelineCanvas
and passed as a prop to TimelineClip instead of recomputing per clip.

Remove dead PlaybackAdapter re-export from useTimelinePlayer — all consumers
import directly from playbackTypes.

packages/studio/src/player/lib/mediaProbe.ts

@@ -0,0 +1,68 @@
+import { Input, UrlSource, ALL_FORMATS } from "mediabunny";
+
+export interface MediaProbeResult {
+  duration: number;
+  width?: number;
+  height?: number;
+  hasVideo: boolean;
+  hasAudio: boolean;
+}
+
+const cache = new Map<string, MediaProbeResult>();
+const inflight = new Map<string, Promise<MediaProbeResult | null>>();
+
+function normalizeUrl(url: string): string {
+  try {
+    return new URL(url, window.location.href).href;
+  } catch {
+    return url;
+  }
+}
+
+async function probeOne(url: string): Promise<MediaProbeResult | null> {
+  const input = new Input({
+    source: new UrlSource(url),
+    formats: ALL_FORMATS,
+  });
+  try {
+    const duration = await input.getDurationFromMetadata();
+    if (duration == null || !Number.isFinite(duration) || duration <= 0) return null;
+
+    const videoTrack = await input.getPrimaryVideoTrack();
+    const audioTracks = await input.getAudioTracks();
+
+    const result: MediaProbeResult = {
+      duration,
+      width: videoTrack?.displayWidth,
+      height: videoTrack?.displayHeight,
+      hasVideo: videoTrack != null,
+      hasAudio: audioTracks.length > 0,
+    };
+    return result;
+  } catch {
+    return null;
+  } finally {
+    input.dispose();
+  }
+}
+
+export function getCachedProbe(url: string): MediaProbeResult | undefined {
+  return cache.get(normalizeUrl(url));
+}
+
+export async function probeMediaUrl(url: string): Promise<MediaProbeResult | null> {
+  const key = normalizeUrl(url);
+  const cached = cache.get(key);
+  if (cached) return cached;
+
+  let pending = inflight.get(key);
+  if (pending) return pending;
+
+  pending = probeOne(key).then((result) => {
+    inflight.delete(key);
+    if (result) cache.set(key, result);
+    return result;
+  });
+  inflight.set(key, pending);
+  return pending;
+}