[umiyosh]

Summary

When the film is longer than the BGM track, the music now carries on as the same piece instead of restarting from its intro with a hard cut.

Closes #32. Part of #16.

Changes

• scripts/audio.ts
  • mediaSeconds(file) reads a file's length from ffmpeg's Duration line (no decoding)
  • bgmLoop() builds the BGM inputs and filters. If the track is long enough, nothing changes. If it is short, the first pass stops before the last 6 s (the closing bars fade out), then the track returns to 30 s (a quarter in for short tracks), past the intro, with a 3 s crossfade, as many times as the film needs. A track under 18 s, too short to crossfade, still loops from its start
  • mix uses it; sound effect input numbers follow the BGM inputs
• Each return reads the file again as its own input (-ss 30 -t … -i bgm). The first version split one input with asplit and crossfaded the branches: it worked with ffmpeg 8.1 but with the bundled ffmpeg 6.0 the music stopped at the first crossfade (silent BGM from 2:50 in a 194 s film). Leaving out the closing bars came from measuring the next version: crossfading the track's fade-out made the music thin for about 4 s

Test plan

• make check: lint, format, typecheck, 107 tests with the coverage gate, quality gates
• Unit: bgmLoop for a long-enough track, one return (178 s track, 194 s film), many returns (40 s, 150 s), a too-short track; mediaSeconds reads Duration and fails without it
• With the bundled ffmpeg: a 20 s tone in a 40 s film plays to the end (the asplit version stops at 14 s, so this test catches it)
• On the real MP4 of the 194 s film with a 178 s BGM: the BGM alone (final audio minus the same mix with BGM volume 0) stays at -27 to -29 dB to the end, against -41 to -48 dB (the quiet intro) after the restart in the old version; the crossfade is 3-5 dB softer for about 2 s
• Listened: the join sounds natural
• CI