Add SourceOS boot plan control metadata

[mdheller]

Summary

Evolves the safe nlboot planner so emitted boot plans carry SourceOS control-plane semantics without adding any host mutation.

Scope

• Adds policy_ref to emitted BootPlan.
• Adds allowed_operations by boot mode.
• Adds proof_requirements by boot mode.
• Adds offline_fallback posture by boot mode.
• Keeps execute=false.
• Keeps existing signed-manifest and one-time enrollment token validation.
• Updates CLI smoke test to assert the new recovery plan metadata.
• Updates README with the new plan fields and fixture names.

Non-goals

• Does not download artifacts.
• Does not write disks.
• Does not call kexec.
• Does not mutate a host.
• Does not change signed example manifest payloads.

Validation

Expected:

make validate