fix(producer): scale frame coverage by playback rate

[miguel-heygen]

Summary

Valid slow-playback clips no longer fail the extracted-frame coverage gate. Coverage now compares delivered frames against the unique source-frame window consumed by the authored slot, using the same normalized playback rate as extraction, while retaining the existing source-duration ceiling and incomplete-extraction protection.

Fixes reported:1787952018.741109.

• https://slack.com/archives/C0BGC335AQY/p1787952018741109
• Related: https://slack.com/archives/C0BGC335AQY/p1787865428565129
• Related: https://slack.com/archives/C0BGC335AQY/p1787902069198369

Test plan

• packages/producer/src/services/render/videoFrameCoverage.test.ts (39 passed)
• Producer typecheck
• oxlint and oxfmt --check on changed files
• Exact Terra fixture with the default 95% gate: extracts 60 frames, expects 60, reports minimum coverage 1, and produces a valid 4.000s MP4 without HF_VIDEO_COVERAGE_THRESHOLD=0

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[miga-heygen]

Reviewed at exact head. Correct fix — a slowed slot at playbackRate 0.5 consumes half the source media, so expectedFrames must scale by playbackRate. Uses the existing normalizePlaybackRate from core. New test verifies the math (4s slot at 0.5x → 60 source frames at 30fps, ratio 1.0).

— Miga