feat(transfer): support MCAP-only scanning and upload readiness

[05F3759DF]

Context

axon_transfer currently uses the presence of a matching JSON sidecar as the default signal that an MCAP recording is finalized and ready to upload. Once sidecar JSON is optional, transfer scanning needs a completion signal that does not depend on a .json file.

Scope

• Support scanning completed MCAP recordings when no JSON sidecar is present.
• Add or reuse a completion signal such as recorder state, a .done marker, SQLite state, or atomic final filename convention.
• Preserve compatibility with existing require_json_sidecar deployments.
• Ensure partially written MCAP files are not uploaded.
• Update configuration and documentation for MCAP-only transfer mode.

Acceptance Criteria

• Transfer can discover completed MCAP recordings without a JSON sidecar when configured for MCAP-only mode.
• Transfer does not enqueue partially written recordings.
• Existing JSON-sidecar-required behavior remains available.
• Tests cover MCAP+JSON, MCAP-only complete, MCAP-only incomplete, and legacy require_json_sidecar behavior.

Depends On

• feat(metadata): make recorder sidecar JSON generation optional #96

Related

• feat(metadata): extract recording index metadata from MCAP keystone#71

[Nauyix]

Completed in #121, building on the existing transfer scanner implementation.

The transfer path now has coverage for the MCAP-only readiness behavior requested here:

• Existing require_json_sidecar=true behavior remains available and continues to require the JSON sidecar.
• MCAP-only mode discovers completed recordings through the configured completion marker.
• Incomplete MCAP-only recordings without the marker are skipped.
• Marker freshness is checked against the MCAP mtime to avoid enqueueing partially written recordings.
• Custom completion marker suffix behavior is covered directly by scanner tests.

Validation completed:

• test_file_scanner and test_transfer_config pass locally.
• New scanner tests cover legacy JSON-sidecar priority even when .mcap.done exists, and custom marker suffix discovery in MCAP-only mode.
• make format-ci, REUSE compliance, and GitHub engineering CI are green for PR test(transfer): cover mcap marker scan cases #121.

Closing as completed.