fix(transform): preserve declared empty camera channels

[harshitethic]

Summary

• keep declared camera channels in the canonical MCAP even when they contain zero messages
• register those channels with the canonical CompressedVideo schema without invoking ffmpeg
• add a regression proving the empty camera survives and changes canonical bytes
• update converter docs to match the preserved-channel behavior

This keeps camera handling consistent with the existing rule that a declared channel is present even when it is empty, and prevents distinct source declarations from collapsing to the same canonical representation.

Fixes #528.

Testing

Focused regression coverage is included in tests/test_transform.py. Tests were not executed through this GitHub connector session.

[github-actions]

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[greptile-apps]

Confidence Score: 4/5

[Medium risk] Changes how empty camera channels are handled in data transformation.

The PR is not safe to merge until the media step skips empty cameras.

Findings

1.  Empty camera breaks media step ▶

Summary

The transform now keeps declared compressed-image camera channels even when they have no messages, so their presence affects canonical output identity.

• Empty cameras are registered with the canonical protobuf video schema without image decoding or transcoding.
• The regression test checks that an empty camera survives and changes the output bytes; the converter guide now describes this behavior.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Empty camera declaration] --> B[Canonical video channel]
  B --> C[Episode.cameras]
  C --> D[Contact-sheet step]
  D --> E[Frame-rate error]

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_{Reviews (1) · Last reviewed commit: "docs(transform): document empty camera p..."}

[greptile-apps]

Empty camera breaks media step

When a source declares an empty camera, this change adds it to Episode.cameras. The built-in contact-sheet step then calls Episode.frames() for it. Episode.video() raises because there are no timestamps to estimate a frame rate, so the media step fails. Keep the declaration, but skip cameras with no frames when making contact sheets.