fix(render): re-encode audio across segment joins

[a27851305-hue]

Summary

• preserve stream-copy for the video track during segment concat
• re-encode AAC audio once into a continuous 48 kHz, 192 kbps stream to avoid per-segment encoder priming/padding clicks
• add a regression test for the generated ffmpeg command

This addresses the audio-click portion of #162. The larger automatic boundary-QC proposal remains separate.

Testing

• python -m unittest discover -s tests -v (17 tests passed)

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Summary by cubic

Re-encodes audio during segment concat to fix clicks from per-segment AAC encoder priming/padding, while preserving lossless video copy. Previously the concat used -c copy for every track; now audio is re-encoded into one continuous 48 kHz, 192 kbps AAC stream. Adds a regression test for the generated ffmpeg command.

^{Written for commit 6ad7a2f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}