v0.1.3 — fix quickstart --demo: bake audio into demo video + sharpen drop transient

Second friend-feedback patch. dj-edit quickstart --demo now actually completes
end-to-end with a detected drop and a rendered per-drop clip.

Fixed

• demo_video.mp4 had no audio track. dj-edit sync requires camera scratch
  audio to cross-correlate against the master — without it, no offsets, empty
  EDL, zero clips. Fix: bake the demo audio INTO the demo video so sync resolves
  trivially.
• Synthesized drop didn't trigger detection. The previous demo "drop" at
  t=15s was a slow volume ramp, which librosa's onset_strength saw as nothing
  remarkable. Fix: regenerate demo_audio.wav with a sharp sub-bass impact at
  t=15s plus a noise impulse for the onset trigger. Analyze now finds:
  build @ 7s + drop @ 15s (intensity 1.0).

Result

dj-edit quickstart --demo now produces in 8.6s:

• set_9x16.mp4 + set_16x9.mp4 (full 30s timeline)
• highlight_9x16.mp4 (drop reel)
• drops/drop_01_t00m15s_int100.mp4 (timestamp + intensity in filename)
• drops/INDEX.txt (tracklist)

A proper smoke test of the install.

Upgrade

brew upgrade dj-edit