refactor(spf): make updateMediaSourceDuration the sole duration writer

syncLiveSeekableRange dropped its defensive mediaSource.duration = Infinity
write — it duplicated updateMediaSourceDuration (the canonical owner). Per the
W3C MSE spec, setLiveSeekableRange requires only readyState === 'open', not a
set duration, so this behavior never needed it. Verified live (Mux LL-HLS):
with the write removed, duration is Infinity and the initial seek lands at the
holdback from the first frame — updateMediaSourceDuration's async write resolves
fast at startup (buffers idle), ahead of seekToLiveEdge's seek.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/spf/src/playback/behaviors/dom/sync-live-seekable-range.ts

@@ -11,10 +11,11 @@
  * so the range is declared before that behavior seeks the playhead into it (a
  * seek outside `seekable` is clamped).
  *
- * Out of scope (deliberate, tracked as follow-ups): `clearLiveSeekableRange()`
- * on the live→ended transition; and dropping the defensive `duration` write
- * below once duration ownership/ordering with `updateMediaSourceDuration` (the
- * canonical, but asynchronous, duration writer) is resolved.
+ * Duration is owned solely by `updateMediaSourceDuration`; this behavior only
+ * declares the seekable range (`setLiveSeekableRange` requires only
+ * `readyState === 'open'` per the W3C MSE spec, not a set `duration`). Out of
+ * scope (tracked follow-up): `clearLiveSeekableRange()` on the live→ended
+ * transition.
  */
 import type { Behavior } from '../../../core/composition/create-composition';
 import { effect } from '../../../core/signals/effect';
@@ -49,15 +50,11 @@ function syncLiveSeekableRangeSetup({
     if (!mediaSource || mediaSource.readyState !== 'open' || !liveWindow) return;
 
     try {
-      // A live seekable range needs a set duration. `updateMediaSourceDuration`
-      // is the canonical duration owner, but writes asynchronously — so guard
-      // here so this doesn't race ahead of it. (Follow-up: resolve ownership.)
-      if (Number.isNaN(mediaSource.duration)) mediaSource.duration = Number.POSITIVE_INFINITY;
       // Re-declared as the window slides so seekable tracks the live window
       // (the full DVR range remains seekable; seek-to-live-edge starts near the edge).
       mediaSource.setLiveSeekableRange(liveWindow.start, liveWindow.end);
     } catch {
-      // readyState raced closed, or duration set rejected — retried on the next window change.
+      // readyState raced closed — retried on the next window change.
     }
   });
 }

packages/spf/src/playback/behaviors/dom/tests/sync-live-seekable-range.test.ts

@@ -55,13 +55,12 @@ function run(opts: { presentation?: MaybeResolvedPresentation; trackId?: string;
 }
 
 describe('syncLiveSeekableRange', () => {
-  it('declares the full live window as seekable and sets Infinity duration', () => {
+  it('declares the full live window as seekable', () => {
     const ms = fakeMediaSource();
     const cleanup = run({ presentation: makePresentation(), trackId: 'v-1', mediaSource: ms });
 
     // [first.startTime, last.startTime + last.duration] = [100, 110].
     expect(ms.setLiveSeekableRange).toHaveBeenCalledWith(100, 110);
-    expect(ms.duration).toBe(Number.POSITIVE_INFINITY);
 
     cleanup();
   });
@@ -96,13 +95,13 @@ describe('syncLiveSeekableRange', () => {
     cleanup();
   });
 
-  it('does not overwrite an already-set (non-NaN) duration', () => {
+  it('leaves duration alone — owned by updateMediaSourceDuration', () => {
     const ms = fakeMediaSource();
-    ms.duration = 500; // e.g. updateMediaSourceDuration already wrote it
     const cleanup = run({ presentation: makePresentation(), trackId: 'v-1', mediaSource: ms });
 
-    expect(ms.duration).toBe(500);
+    // Declares the range without touching duration (still NaN from the fake).
     expect(ms.setLiveSeekableRange).toHaveBeenCalledWith(100, 110);
+    expect(ms.duration).toBeNaN();
 
     cleanup();
   });