fix(runtime): clear play guard after hard seek to prevent audio desync on scrub (#639)

* fix(runtime): clear play guard after hard seek to prevent audio desync on scrub

When scrubbing the timeline during playback, syncRuntimeMedia detects
the offset jump and hard-seeks the media element. But the in-flight
play() guard (playRequested WeakSet) from the previous play() call
prevented the next sync tick from re-issuing play() — leaving the
element paused at the new position for 50-150ms while the GSAP timeline
continued advancing. This caused audible audio desync after every scrub.

Fix: clear playRequested on the element after a hard seek so the very
next sync tick can re-issue play().

Also adds a lint rule (video_audio_double_source) that catches
compositions where an unmuted <video> and a separate <audio> point to
the same source — a pattern that causes double playback at runtime.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(runtime): detect failed seeks past MP3 buffer and force full fetch

Root cause: streaming MP3 with preload="metadata" only buffers the first
~15 seconds. Seeking past the buffered range silently fails — currentTime
stays at 0 while the timeline advances, causing permanent audio desync
that only a page refresh fixes.

Three changes:
1. Move preload="auto" enforcement to run for ALL active elements on
   every sync tick (not just during play). This catches elements whose
   preload was overridden after init.ts set it.
2. After a hard seek, check if currentTime actually reached the target.
   If not (drift > 0.5s), call load() once to force the browser to
   fully fetch the media and build a complete seek index.
3. Clear the load-retry guard when the clip leaves its active window
   so re-entry can retry if needed.

Reproduced on hyperframes.dev Hermes launch video: vo.mp3 buffered to
15.96s, seeking to 20s failed silently. bg-music.wav (fully buffered)
was unaffected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/src/lint/rules/media.ts

@@ -460,6 +460,44 @@ export const mediaRules: Array<(ctx: LintContext) => HyperframeLintFinding[]> =
     return findings;
   },
 
+  // video_audio_double_source — catches audible <video> paired with a separate
+  // <audio> pointing to the same file, which causes double playback at runtime
+  ({ tags }) => {
+    const findings: HyperframeLintFinding[] = [];
+    const videoSources = new Map<string, { id?: string; raw: string }>();
+    const audioSources = new Map<string, { id?: string; raw: string }>();
+
+    for (const tag of tags) {
+      if (!readAttr(tag.raw, "data-start")) continue;
+      const src = readAttr(tag.raw, "src");
+      if (!src) continue;
+      const elementId = readAttr(tag.raw, "id") || undefined;
+      if (tag.name === "video") {
+        const isMuted = hasAttrName(tag.raw, "muted");
+        if (!isMuted) {
+          videoSources.set(src, { id: elementId, raw: tag.raw });
+        }
+      } else if (tag.name === "audio") {
+        audioSources.set(src, { id: elementId, raw: tag.raw });
+      }
+    }
+
+    for (const [src, audioInfo] of audioSources) {
+      const videoInfo = videoSources.get(src);
+      if (!videoInfo) continue;
+      findings.push({
+        code: "video_audio_double_source",
+        severity: "error",
+        message: `<audio${audioInfo.id ? ` id="${audioInfo.id}"` : ""}> and <video${videoInfo.id ? ` id="${videoInfo.id}"` : ""}> both point to the same source. The unmuted video already provides audio — the duplicate <audio> will cause double playback and echo.`,
+        elementId: audioInfo.id,
+        fixHint:
+          "Either mute the video (add `muted` attribute) and keep the separate <audio>, or remove the <audio> element and let the video provide its own audio track.",
+        snippet: truncateSnippet(audioInfo.raw),
+      });
+    }
+    return findings;
+  },
+
   // imperative_media_control
   findImperativeMediaControlFindings,
 ];