Audio-clocked playback for judging sync

Lip sync cannot be judged silently. extract.sh now pulls the audio track and
writes manifest.json alongside the frames; the page reads the true extraction
rate from it rather than assuming one, since a guessed fps desynchronises
picture from sound - the one thing this view exists to show.

Audio is the clock: frame = floor(currentTime * fps). A slow render loop drops
frames instead of drifting, and half/quarter speed work via playbackRate with
the picture following for free. Scrubbing, stepping and clicking a thumbnail
all seek the audio too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extract.sh

@@ -1,17 +1,38 @@
 #!/usr/bin/env bash
-# Extract a clip to a PNG sequence for the take builder.
+# Extract a clip to a PNG sequence + audio + manifest for the take builder.
 #
 # Frames are pre-extracted rather than decoded in the page on purpose: browser
 # video seeking by currentTime is approximate and requestVideoFrameCallback only
 # delivers frames at playback speed, so neither gives a deterministic per-frame
 # pass. A PNG sequence is exact, instantly seekable, and reproducible.
+#
+# Audio comes out alongside because the page uses it as the PLAYBACK CLOCK -
+# frame = floor(audio.currentTime * fps) - so picture and sound cannot drift
+# apart no matter how long the shot is or how slow the render loop runs.
 set -euo pipefail
 
 src="${1:?usage: ./extract.sh CLIP [FPS] [OUTDIR]}"
-fps="${2:-24}"
+fps="${2:-12}"
 out="${3:-frames}"
 
-rm -rf "$out"
-mkdir -p "$out"
+rm -rf "$out"; mkdir -p "$out"
 ffmpeg -hide_banner -loglevel warning -i "$src" -vf "fps=$fps" "$out/%04d.png"
-echo "$(ls -1 "$out" | wc -l) frames at ${fps}fps -> $out/"
+count=$(ls -1 "$out" | wc -l)
+
+# Mono is enough for judging sync and halves the file. Absent audio is not fatal.
+if ffprobe -v error -select_streams a:0 -show_entries stream=codec_type \
+     -of csv=p=0 "$src" 2>/dev/null | grep -q audio; then
+  ffmpeg -hide_banner -loglevel warning -y -i "$src" -vn -ac 1 -ar 44100 audio.wav
+  audio='"audio.wav"'
+  echo "audio -> audio.wav"
+else
+  audio='null'
+  echo "no audio stream"
+fi
+
+# The page must know the true extraction rate: if it guessed, audio and picture
+# would drift. Source of truth lives here, next to the frames it describes.
+printf '{"fps":%s,"frames":%s,"dir":"%s","audio":%s,"source":"%s"}\n' \
+  "$fps" "$count" "$out" "$audio" "$(basename "$src")" > manifest.json
+
+echo "$count frames at ${fps}fps -> $out/  (manifest.json written)"