Add nlboot-compatible adapter skeleton

[mdheller]

Summary

Adds the first executable adapter boundary for nlboot-compatible SourceOS boot flows.

What changed

• Adds sourceos_boot.adapter with:
  • DeviceClaim
  • BootAuthorization
  • BootEvidence
  • SourceOSBootAdapter
• Models the first pure control-plane handshake:
  • announce
  • authorize
  • fetch manifest
  • verify
  • emit evidence
• Adds smoke coverage for announce/fetch/evidence construction.

Why

This starts turning BootReleaseSet v1 from a contract into an implementation path while keeping the first step side-effect-free and safe for CI.

Next

• Inspect SociOS-Linux/nlboot implementation details.
• Map actual nlboot request/response fields to these SourceOS objects.
• Add manifest digest verification and signed trust checks.