docs: add real React 19 useRef drift lab

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What changed

• add a reproducible React 18 → 19 useRef contract-drift lab
• pin and verify @types/react 18.3.12 and 19.0.1
• install with dependency lifecycle scripts disabled
• expose the case in the README, GitHub Pages landing page, and llms.txt
• add regression coverage for versions, safety controls, and public discovery

The breaking change is documented in the official React 19 upgrade guide.

Validation

• npm run demo:react-19: React 18 PASS, React 19 contract_mismatch reproduced
• npm run verify: 97 tests passed and the full release gate passed
• No generated evidence or dependency files are committed

Trust impact

This adds a source-repository demonstration only. It does not change Receipt semantics, check policy, network behavior, or runtime-correctness claims.

AI assistance

OpenAI Codex assisted with implementation, tests, documentation, and local validation; all changes and outputs were reviewed locally.