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/hooks/useTimelinePlayer.ts

@@ -7,7 +7,7 @@ import { useTimelineSyncCallbacks } from "./useTimelineSyncCallbacks";
 // Re-export public API consumed by tests and external modules.
 // All of these were previously defined in this file; they now live in focused
 // sub-modules but are re-exported here so existing import sites don't change.
-export type { PlaybackAdapter, ClipManifestClip } from "../lib/playbackTypes";
+export type { ClipManifestClip } from "../lib/playbackTypes";
 export { createStaticSeekPlaybackAdapter } from "../lib/playbackAdapter";
 export {
   getTimelineElementSelector,
@@ -42,6 +42,7 @@ import {
   setPreviewPlaybackRate,
   shouldMutePreviewAudio,
 } from "../lib/timelineIframeHelpers";
+import { probeMediaUrl, getCachedProbe } from "../lib/mediaProbe";
 
 // ---------------------------------------------------------------------------
 // Hook
@@ -106,6 +107,31 @@ export function useTimelinePlayer() {
       if (!state.timelineReady) {
         setTimelineReady(true);
       }
+
+      // Asynchronously enrich media elements missing sourceDuration via mediabunny.
+      // The probe reads file headers only — no full decode — so this is cheap.
+      const needsProbe = mergedElements.filter(
+        (el) =>
+          el.src &&
+          el.sourceDuration == null &&
+          ["video", "audio"].includes(el.tag.toLowerCase()) &&
+          !getCachedProbe(el.src),
+      );
+      if (needsProbe.length > 0) {
+        void Promise.allSettled(
+          needsProbe.map(async (el) => {
+            const result = await probeMediaUrl(el.src!);
+            if (!result) return;
+            const current = usePlayerStore.getState().elements;
+            const key = el.key ?? el.id;
+            const idx = current.findIndex((e) => (e.key ?? e.id) === key);
+            if (idx === -1 || current[idx].sourceDuration != null) return;
+            const patched = current.slice();
+            patched[idx] = { ...current[idx], sourceDuration: result.duration };
+            setElements(patched);
+          }),
+        );
+      }
     },
     [setElements, setTimelineReady, setDuration],
   );