feat(metadata): expose sidecar JSON as a derived compatibility view

[05F3759DF]

Context

Some existing tools and downstream workflows expect sidecar JSON. Once Axon sidecars become optional, Keystone should provide sidecar-equivalent JSON as a derived view from indexed MCAP metadata rather than requiring robots to store and upload a separate long-lived JSON file.

Scope

• Add an API or export path that returns sidecar-equivalent JSON derived from Keystone's MCAP metadata index.
• Match the existing sidecar schema for compatibility where practical.
• Include checksum fields from Keystone upload verification state rather than MCAP metadata.
• Document source-of-truth rules for MCAP metadata, Keystone checksum state, and legacy JSON sidecars.
• Preserve compatibility for older recordings that only have uploaded JSON sidecars.

Acceptance Criteria

• Keystone can return sidecar-equivalent JSON for MCAP-only recordings.
• The derived JSON includes task, device, recording, topic summary, and checksum fields when available.
• Legacy MCAP+JSON recordings remain readable through the same API.
• The API clearly distinguishes missing optional metadata from extraction failures.
• Tests cover MCAP-only derived JSON, legacy JSON compatibility, checksum inclusion, and missing-field behavior.

Depends On

• feat(metadata): extract recording index metadata from MCAP #71
• feat(upload): manage MCAP checksum verification and stored hash #72

Related

• feat(metadata): ensure MCAP contains sidecar-equivalent metadata axon#95
• feat(metadata): make recorder sidecar JSON generation optional axon#96
• feat(uploader): compute upload checksum without requiring sidecar JSON axon#98