Add NLBoot integration docs, fixtures, Makefile validation, and maturity record

[Copilot]

sourceos-boot had no dedicated integration guide for its nlboot adapter, no Makefile validation target (required by AGENTS.md when adding fixtures), and no repo.maturity.yaml.

Added

• docs/NLBOOT_INTEGRATION.md — operational integration guide covering the 5-stage adapter flow (announce → authorize → fetch → verify → evidence), boot mode→channel→action mapping table, CLI usage, fixture inventory, and known gaps (signature verification not yet wired, artifact fetch/cache lives in Rust lane, host mutation gated)
• examples/nlboot/adapted-output.example.json — concrete NlbootAdapterOutput fixture produced by running adapt-nlboot against the existing manifest/token fixtures; shows exact authorization, BootReleaseSet patch, and evidence envelope structure
• Makefile — validate target (BootReleaseSet schema validation + nlboot adapter CLI smoke test) and test target (pytest), satisfying the AGENTS.md requirement
• repo.maturity.yaml — M2 maturity record with integration links to nlboot, sourceos-spec, and downstream consumers, plus explicit next actions
• .gitignore — excludes __pycache__, .pyc, .pytest_cache, build artifacts

CLI output shape (NlbootAdapterOutput)

{
  "apiVersion": "sourceos.dev/v1",
  "kind": "NlbootAdapterOutput",
  "authorization": { "bootReleaseSetRef": "boot/demo", "tokenId": "token-demo-1", ... },
  "bootReleaseSetPatch": {
    "channels": ["recovery"],
    "artifacts": [{ "role": "kernel", ... }, ...],
    "signature": { "type": "x509", "bundleRef": "urn:srcos:signature:demo", ... },
    "provenance": { "builderId": "trusted-key-1", "attestations": ["slsa", "in-toto"], ... }
  },
  "evidence": { "selectedChannel": "recovery", "bootMode": "recovery", "reports": [...], ... }
}

No code changes; documentation, fixtures, and build metadata only.

[mdheller]

Approved. Scope matches issue #11: NLBoot integration docs, fixture, Makefile validation, and maturity metadata only. CI is green and no host-changing behavior was introduced.