Provide a versioned USB serial command and status interface

[PurpleSentinel]

Parent: #23

Summary

Expose device identity, health, settings, live state, and safe control operations to host tools over USB.

Acceptance criteria

• Define a documented, versioned request and response format.
• Keep normal standalone operation working with no host attached.
• Validate all writes and reject commands that are unsafe during a live session.
• Avoid mixing protocol output with unstructured debug messages.
• Provide host-side protocol tests and an example client.

Roadmap group

Data, configuration, and maintenance