Fix half-rate MXF video downscale

[bjay-wk]

Summary

• Detect MXF MPEG-2 sources whose probed video timing reports r_frame_rate at half of avg_frame_rate, matching the PC-... sample pattern.
• During local downscale, extract the video essence before transcoding and force the corrected output frame rate so the downscaled MP4 keeps the expected duration and frame count.
• Bypass stale rendition-cache reuse for detected half-rate sources and add unit coverage for the detection path plus the Avid-style metadata case that should not be corrected.

Validation

• cargo test media::transcode
• cargo test
• Manual ffmpeg check against V...: extracted 1573 video frames and downscaled to a 25 fps, 62.92s, 1280x720 MP4.

Note: the temporary clean worktree used for the PR branch could not run Cargo because generated/tellers_api_client is not present in that worktree; validation was run from the primary workspace where the generated dependency exists.