feat: add NLBoot object schemas, examples, and validation

[Copilot]

Adds canonical JSON Schema (draft 2020-12) definitions for four NLBoot-related object types, making this repo the normative contract home for boot plan, artifact cache, proof, and Apple Silicon adapter semantics.

New schemas

Schema	URN prefix	Description
NLBootPlan	urn:srcos:nlboot-plan:	Ordered boot stages with artifact refs and verification policy
ArtifactCacheRecord	urn:srcos:artifact-cache:	Content-addressed local artifact cache entry with kind, hash, status, platform, and TTL
BootProofRecord	urn:srcos:boot-proof:	Immutable per-boot integrity proof linking plan, per-stage verdicts, and attestation evidence
AppleSiliconAdapterEvidence	urn:srcos:as-adapter-evidence:	Asahi adapter evidence: chip model, security policy, m1n1/U-Boot versions, boot-chain hashes, boot-picker entry

All schemas follow v2 conventions (additionalProperties: false, id/type/specVersion discriminators, description on every property).

Supporting changes

• Examples — four conforming payloads using the M2 demo device as a coherent fixture, cross-referencing existing urn:srcos: URNs
• Validation — tools/validate_nlboot_examples.py + make validate-nlboot-examples target; all 4 pairs pass
• OpenAPI — Boot tag + POST endpoints for all four types
• Hydra/JSON-LD — type aliases and hydra:supportedClass blocks for all four types
• ADR — docs/adr/0011-nlboot-object-schemas.md documents scope boundaries (no runtime implementation in this repo)
• Catalog — schemas/README.md quick-reference rows + new Boot / NLBoot family section; examples/README.md index rows; CHANGELOG.md entry

Non-goals preserved

• No NLBoot runtime or boot-chain implementation code
• No changes to control-plane schemas or legacy wrappers

[mdheller]

Approved. Scope matches issue #68: canonical NLBoot object schemas, examples, validation tool, ADR, and catalog/OpenAPI metadata. No runtime implementation or boot-chain behavior is introduced. No workflow run is visible on the branch, but the PR body includes validation evidence for the schema/example pairs.