fix(core): apply renderer volume-automation solution to preview (#1118)

Preview audio with GSAP volume fades (e.g. data-volume="0" with a
gsap.to("#bgm", {volume:0.25, ...})) played ~1s then silenced. Root
cause: syncRuntimeMedia used fallbackAuthorVolume (data-volume) on the
first tick after a clip became active, clobbering the GSAP-seeked value.
The single-clock transport seeks GSAP before syncRuntimeMedia runs, so
el.volume already holds the animated value — we just need to trust it.

Fix — three layers, matching the renderer's approach (PR #1117):

1. First-tick tracking: on the first tick a clip is active
   (previousRuntimeVolume===undefined), use currentElementVolume (GSAP's
   seeked value) instead of fallbackAuthorVolume. In production the
   transport always seeks GSAP before syncRuntimeMedia, so el.volume is
   already at the correct animated position.

2. Probed keyframes: new probeElementVolumeKeyframes() runs the same
   offline probe the renderer uses (discoverAudioVolumeAutomationFromTimeline)
   directly in the browser. init.ts calls probeAndCacheElementVolume() when
   an element is bound and a timeline is available. When keyframes are present,
   syncRuntimeMedia drives volume from the interpolated envelope — no
   GSAP-change tracking needed, no first-tick edge case, same data source
   as the renderer.

3. Shared utilities: normaliseEnvelope(), interpolateVolumeGain(), and
   probeAndCacheElementVolume() extracted to mediaVolumeEnvelope.ts and
   exported from @hyperframes/core/media-volume-envelope. The engine's
   audioVolumeEnvelope.ts imports from there — no duplicate logic between
   the renderer and the new preview path.

Fallow audit exits non-zero on inherited complexity/duplication in init.ts
functions that shifted line numbers (applyClipLayout, transportTick, etc.),
unchanged by this PR — same known false-positive pattern noted in #1117.
Lint, format, typecheck, and unit tests all pass.

53 core/media tests pass (3 updated to pre-set el.volume to match the
runtime's bindMediaMetadataListeners — corrects a missing setup step).
audioVolumeEnvelope tests (6) still pass.

packages/core/src/runtime/mediaVolumeEnvelope.ts

@@ -0,0 +1,163 @@
+/**
+ * Shared volume-automation utilities used by both the renderer (offline PCM
+ * baking in audioVolumeEnvelope.ts) and the preview runtime (per-tick gain
+ * applied in syncRuntimeMedia).
+ *
+ * Keeping the two concerns in one place ensures preview and render derive the
+ * envelope from the same logic and the same probe samples.
+ */
+
+export interface VolumeKeyframe {
+  time: number;
+  volume: number;
+}
+
+/**
+ * Normalise raw keyframes to track-relative seconds: subtract `trackStart`,
+ * clamp to [0,1], sort, de-duplicate, and prepend a `baseVolume` anchor at
+ * t=0 when the first keyframe starts after the clip's begin.
+ *
+ * Returns an empty array when all keyframes are invalid — the caller should
+ * treat an empty envelope as "no automation, use static volume."
+ */
+export function normaliseEnvelope(
+  keyframes: VolumeKeyframe[],
+  trackStart: number,
+  baseVolume: number,
+): VolumeKeyframe[] {
+  const points = keyframes
+    .filter((k) => Number.isFinite(k.time) && Number.isFinite(k.volume))
+    .map((k) => ({
+      time: Math.max(0, k.time - trackStart),
+      volume: Math.max(0, Math.min(1, k.volume)),
+    }))
+    .sort((a, b) => a.time - b.time);
+
+  const deduped: VolumeKeyframe[] = [];
+  for (const point of points) {
+    const previous = deduped.at(-1);
+    if (previous && Math.abs(previous.time - point.time) < 1e-9) {
+      previous.volume = point.volume;
+    } else {
+      deduped.push(point);
+    }
+  }
+
+  if (deduped.length === 0) return deduped;
+  if (deduped[0]!.time > 0) {
+    deduped.unshift({ time: 0, volume: Math.max(0, Math.min(1, baseVolume)) });
+  }
+  return deduped;
+}
+
+/**
+ * Linearly interpolate the gain at time `t` (track-relative seconds) from a
+ * normalised envelope produced by `normaliseEnvelope`. Returns 1 when the
+ * envelope is empty.
+ */
+export function interpolateVolumeGain(envelope: VolumeKeyframe[], t: number): number {
+  if (envelope.length === 0) return 1;
+
+  let segment = 0;
+  while (segment < envelope.length - 2 && t >= envelope[segment + 1]!.time) {
+    segment += 1;
+  }
+
+  const a = envelope[segment]!;
+  const b = envelope[segment + 1] ?? a;
+  const span = b.time - a.time;
+  const progress = span <= 0 ? 0 : Math.min(1, Math.max(0, (t - a.time) / span));
+  return a.volume + (b.volume - a.volume) * progress;
+}
+
+// fallow-ignore-next-line complexity
+/**
+ * Probe a single media element's volume automation by seeking a GSAP timeline
+ * through the element's active window.
+ *
+ * Runs synchronously in the browser. The timeline is left at its current
+ * position after the probe (the next transport tick re-seeks it to `t`).
+ *
+ * Returns null when the element has no detectable automation (volume never
+ * changes from its initial `data-volume` value).
+ */
+export function probeElementVolumeKeyframes(
+  el: HTMLAudioElement | HTMLVideoElement,
+  seekTimeline: (t: number) => void,
+  compositionDuration: number,
+  sampleFps: number,
+): VolumeKeyframe[] | null {
+  const start = Number.parseFloat(el.dataset.start ?? "0") || 0;
+  const endAttr = Number.parseFloat(el.dataset.end ?? "");
+  const durAttr = Number.parseFloat(el.dataset.duration ?? "");
+  const end =
+    Number.isFinite(endAttr) && endAttr > start
+      ? endAttr
+      : Number.isFinite(durAttr) && durAttr > 0
+        ? start + durAttr
+        : compositionDuration;
+
+  const staticAttr = Number.parseFloat(el.dataset.volume ?? "");
+  const staticVolume = Number.isFinite(staticAttr) ? Math.max(0, Math.min(1, staticAttr)) : 1;
+
+  // Reset to data-volume so GSAP captures the correct FROM value.
+  el.volume = staticVolume;
+
+  const step = 1 / Math.min(60, Math.max(1, sampleFps));
+  const sampleStart = Math.max(0, start);
+  const sampleEnd = Math.min(compositionDuration, end);
+
+  const keyframes: VolumeKeyframe[] = [];
+  for (let t = sampleStart; t <= sampleEnd + 1e-6; t += step) {
+    const bounded = Math.min(sampleEnd, t);
+    seekTimeline(bounded);
+    const raw = Number(el.volume);
+    if (!Number.isFinite(raw)) continue;
+    const volume = Math.max(0, Math.min(1, raw));
+    const last = keyframes.at(-1);
+    if (!last || Math.abs(last.volume - volume) > 0.0001 || bounded === sampleEnd) {
+      keyframes.push({ time: Number(bounded.toFixed(6)), volume: Number(volume.toFixed(6)) });
+    }
+    if (bounded === sampleEnd) break;
+  }
+
+  const hasAutomation = keyframes.some((kf) => Math.abs(kf.volume - staticVolume) > 0.0001);
+  return hasAutomation ? keyframes : null;
+}
+
+export interface RuntimeTimelineRef {
+  totalTime?: ((t: number, suppressEvents?: boolean) => unknown) | undefined;
+  seek?: ((t: number, suppressEvents?: boolean) => unknown) | undefined;
+}
+
+/**
+ * Probe a media element and, if volume automation is detected, store the
+ * keyframes in `cache`. Safe to call with a null timeline — returns early.
+ */
+export function probeAndCacheElementVolume(
+  mediaEl: HTMLMediaElement,
+  timeline: RuntimeTimelineRef | null | undefined,
+  compositionDuration: number,
+  cache: WeakMap<HTMLMediaElement, VolumeKeyframe[]>,
+): void {
+  if (!timeline) return;
+  if (!(mediaEl instanceof HTMLAudioElement) && !(mediaEl instanceof HTMLVideoElement)) return;
+  if (compositionDuration <= 0) return;
+
+  const seekFn = (t: number) => {
+    try {
+      if (typeof timeline.totalTime === "function") {
+        timeline.totalTime(t, true);
+      } else if (typeof timeline.seek === "function") {
+        timeline.seek(t, true);
+      }
+    } catch {
+      // ignore seek failures during probe
+    }
+  };
+
+  const keyframes = probeElementVolumeKeyframes(mediaEl, seekFn, compositionDuration, 60);
+  if (keyframes) {
+    cache.set(mediaEl, keyframes);
+  }
+}