feat(dna): add modality-aware artifact DNA

[szmyty]

Summary

• add the versioned renderflow.artifact-dna/v1 contract and comparison contract
• add a provider-neutral extractor interface with independent AI, network, remote, source-byte, and observation budgets
• make local deterministic extraction first-class for publication-relevant visual/layout geometry, SVG palettes and structure, and text-layout statistics
• store DNA as an immutable provenance-linked intermediate artifact while preserving source identity and digests
• apply secret, PII, and protected-reference hygiene before extractor output is exposed
• keep heuristic and probabilistic observations as candidates requiring human review
• add explainable, weighted similarity reports that explicitly reject direct-imitation and legal-clearance claims
• add renderflow dna extract, validate, and compare
• add Draft 2020-12 schemas, redistribution-safe visual/sonic/textual/compound fixtures, and user/CLI documentation

This provides the visual/layout-first foundation needed by publication workflows while keeping richer Sonic DNA normalization in #397 and using the model-capability/skill seams from #396 for optional semantic enrichment.

Verification

• changed Rust files formatted and checked with rustfmt --check
• cargo check --workspace --all-features --locked
• cargo clippy --workspace --lib --bins --all-features --locked -- -D warnings -A clippy::too_many_arguments
• cargo test --package renderflow --lib dna::tests --locked — 8 passed
• both Draft 2020-12 schemas checked and all four fixtures validated with jsonschema
• generated DNA and comparison outputs validated against their schemas
• manual CLI smoke test against examples/magazine/assets/cover.svg
• MkDocs site build completed

Intentionally deferred

The full test suite and GitHub Actions were intentionally deferred for this speed-run sprint, per project direction. The focused checks above passed.

Closes #387