@Codex implement release artifact surface contract

[mdheller]

@codex takeover requested.

Open a bounded PR adding the normative release artifact surface contract for model fabric.

Scope:

• add docs/RELEASE_ARTIFACT_SURFACE.md
• add examples/model-fabric-release-artifact.example.json
• add validator and tests
• wire validation into repo-native validation if present

Required concepts:

• development formula
• release artifact formula
• release dry-run manifest
• checksum
• SBOM
• provenance
• promotion state
• rollback ref
• SourceOS carry ref

Acceptance criteria:

• validation passes
• docs define repo ownership for each object
• no fake artifact URLs or checksums

Do not:

• do not publish releases
• do not store binaries or model artifacts