Add SourceOSModelCarryRef carry-layer projection

[mdheller]

Summary

Adds the SourceOSModelCarryRef carry-layer projection path:

• examples/sourceos-model-carry-ref.local-llama32-3b.json
• tools/validate_sourceos_model_carry_refs.py
• docs/sourceos-model-carry-ref.md

Why

This lets sourceos-model-carry consume the new SourceOSModelCarryRef vocabulary from sourceos-spec without overloading the existing SourceOSCarryRef service reference validator or LocalModelProfile validator.

Validation

• Added a stdlib-only validator for SourceOSModelCarryRef projection examples.
• The validator checks the expected type, URN identity, spec version, governance ref, router profile ref, immutable model-state boundary, release-set refs, and fallback refs.

Boundary notes

This does not download models, carry mutable model weights, authorize tuning, grant tool use, or change runtime behavior.