feat: add artifact validation and conformance gates

[szmyty]

Summary

• add a versioned artifact-validator registry with structured validator evidence and native format probes
• gate terminal materialization on required validation and rejected fidelity classes, with branch-local or fatal policy
• generate a machine-readable capability conformance matrix and expose it through renderflow capabilities --matrix
• update run/spec schemas and documentation for validation states, policies, and conformance claims

Verification

• cargo check --workspace --all-targets --locked
• changed Rust files pass rustfmt --check
• git diff --check
• JSON schemas parse successfully
• MkDocs/example YAML syntax parses successfully
• live renderflow capabilities --matrix --format json smoke check

Per the current sprint mode, the broad test suite and CI were intentionally deferred; no test suite was run.

Closes #360

[devactivity-app]

Pull Request Summary by devActivity

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