Add FIPS-ready RSA-PSS manifest verification

[mdheller]

Summary

Refines nlboot boot-manifest validation into a FIPS-ready RSA-PSS/SHA-256 profile.

This PR does not claim FIPS 140-3 certification by itself. FIPS compliance depends on deployment with a validated cryptographic module/runtime. This PR enforces protocol-level requirements needed for that posture:

• signature_algorithm = rsa-pss-sha256
• crypto_profile = fips-140-3-compatible
• trusted RSA public-key input for CLI verification
• RSA-PSS/SHA-256 signature verification before plan construction
• safe boot plans remain execute=false

Key Changes

• Adds verification helpers for trusted RSA keys and canonical manifest payloads.
• Requires FIPS-ready manifest metadata in SignedBootManifest.
• Adds trusted-key fixture and signed recovery manifest example.
• Updates CLI smoke to use --trusted-keys and --require-fips.
• Updates validation to install cryptography.
• Updates tests for required manifest fields and FIPS-ready rejection paths.

Software Review

Correctness: closes the previous signature-shape gap. nlboot now verifies a recovery manifest against a trusted RSA key before producing a boot plan.

Risk: moderate. Adds cryptographic dependency and stricter manifest requirements. It remains side-effect-free: no artifact fetch, disk mutation, or host execution.

Weakness: this is FIPS-ready protocol enforcement, not a certification assertion. Runtime/module validation remains a deployment requirement.