fix(media-use): remove clicks and ~25ms gaps at every transcript-cut splice

[larrylicuanan2488-wq]

The problem

skills/media-use/scripts/transcript-cut.mjs extracts each kept range to its own final-codec file, then concat-copies them. Two audible artifacts result at every cut, and they are separate bugs:

1. No boundary fades. Concat splices raw segment edges together, so the waveform steps discontinuously and each cut clicks.
2. Encoder priming silence. Each segment is encoded to AAC independently, so the encoder pads every one with priming samples; stream-copy concat bakes that in as a silent gap. It also accumulates — the nth splice lands roughly 25n ms late, so splice positions no longer match the sum of the preceding segment durations.

The second survives the first. Adding fades alone still leaves the gap, which is what made it easy to miss.

Reproduce

node skills/media-use/scripts/transcript-cut.mjs \
  --input take.mp4 --transcript take.json --keep "5-10,20-25,40-45" --out out.mp4

Widest run of digital silence within ±0.25 s of each nominal splice:

	splice @5S	splice @10s
main	24.4 ms	24.1 ms
this PR	0.1 ms	0.1 ms

The fix

Intermediates carry PCM (.mkv for video, .wav for audio-only) rather than the final codec, so audio is encoded exactly once — at concat. Per-segment 30 ms afade in/out ramps handle the click, scaled down on segments shorter than 4× the ramp and skipped on degenerate ones.

Not changed

• --copy — stream copy cannot filter, so it still produces final-codec segments and a copy concat. The flag already trades accuracy for speed; this leaves that trade alone.
• Audio-only output — identical duration before and after (10.000000 s on the same two-range cut). .wav intermediates avoid the aac-in-wav timing break the existing encodeArgsFor comment warns about.

Verification

• scripts/lib/*.test.mjs: 34/35. The one failure, lut-preset-provider.test.mjs, reproduces on unmodified main and is unrelated (it reads a packages/core/src/colorGrading.ts that does not exist in this repo).
• Measurements above taken on public-domain footage (Prelinger Archives, The Quiet Revolution, 1956).

🤖 Generated with Claude Code

[miguel-heygen]

The failing check is Skills: manifest in sync. Please regenerate and commit the root manifest with:

bun run --cwd packages/cli gen:skills-manifest

This updates the media-use fingerprint from 8f21655c to 2f32518b. Please push the resulting skills-manifest.json change to this PR.