fix(transform): preserve declared empty camera channels

[harshitethic]

Summary

• preserve declared-but-empty compressed-image camera channels during canonicalization instead of dropping them
• register those channels as empty canonical foxglove.CompressedVideo channels so the source declaration remains part of provenance and content identity
• keep the existing behavior that ffmpeg is not resolved or stamped as used when no camera payload was actually transcoded
• add a regression test proving an empty camera declaration survives the transform and produces a different content-addressed episode from an otherwise identical source without that declaration

Verification

• git diff --check
• uv run ruff check src/hflow/transform.py tests/test_processing_regressions.py
• uv run pytest -q tests/test_processing_regressions.py -k empty_camera

Fixes #528

[github-actions]

👋 Hi @harshitethic — thanks for the contribution! To keep starter issues available
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