Enforce framework required_fields and add systemic drift guards

[davidahmann]

Summary

• add required_fields for every framework control and enforce it in framework loader/schema
• add schema drift guard tests for schemas/v1 <-> core/schema/v1 parity
• add builtin type registry parity tests against core/schema/v1/types/*.schema.json
• update README install commands to resolve latest published release tag dynamically (no hardcoded version)

Validation

• go test ./core/schema/...
• go test ./...
• gait compatibility check with local proof via temporary go.work: go test ./...