fix(engine): sample-accurate volume automation so dense fades keep their audio (#1117)

Animated media volume (GSAP/JS fades) dropped the audio track entirely for dense
fades. The 60 Hz timeline probe emits 100-300 keyframes for a multi-second fade,
which were folded into an FFmpeg `volume` expression nesting one `if(lt(t,...))`
per keyframe. Past ~95 nested levels (build-dependent, lower on some Linux ffmpeg
builds) the expression overflows FFmpeg's evaluator, fails filter-graph init,
fails the whole mix, and the muxer omits audio — so a `data-volume="0"` fade-in
rendered with no audio at all (follow-up to #1066; this is why #1064's own
scenario regressed once the fade was dense enough).

Apply volume automation as sample-accurate gain, layered so audio is never lost:

1. Primary: bake the envelope into the prepared PCM samples in-process
   (audioVolumeEnvelope.ts). The track WAV is always pcm_s16le/48k/stereo;
   multiply its samples by the interpolated envelope and atomically rename the
   result into place, then mix at unity. No expression, no keyframe ceiling,
   exact at every sample, and the downstream ffmpeg amix/AAC encode is untouched
   so golden baselines only change where a fade is applied. The RIFF parser
   scans chunks order-independently and accepts only 16-bit PCM, falling back
   otherwise. The output is written to a random-named sibling and renamed, so a
   crash can't leave a truncated WAV and there's no predictable-path write.
2. Fallback: RDP-bounded ffmpeg `volume` expression (0.5% tolerance, capped at
   32 segments) for the rare case a WAV is not 16-bit PCM. 0.5% keeps the
   rendered envelope within ~0.2 dB of the source curve.
3. Backstop: if an automated mix still fails, retry once at base volume and
   surface the degradation rather than dropping the track.

This mirrors how OSS NLEs render automation (sample-level gain): MoviePy,
Kdenlive/Shotcut (MLT), Remotion.

Verified end-to-end: a 297-keyframe fade that rendered with no audio now bakes
all 297 keyframes sample-accurately. Adds unit tests for sample-accurate gain,
track-start offset, base/tail holds, thousands of keyframes, order-independent
chunk parsing, and format rejection, plus mixer regression tests for bounded
nesting and the base-volume backstop.