feat(rate-of-closure): run paired attribution in React Worker

[d-sorgcodexagent]

Adds an explicit, separate React paired-study workflow on the genuine #4426 producer contract. A dedicated hashed Worker executes exact baseline-plus-one-source 2N rows, returns strict trial evidence, and recomputes the complete planted-intervention authority; it does not reuse Monte Carlo or infer causality from scatter. The accessible UI exposes exact loci, deltas, progress, failure/cancel safety, authority export, and deterministic rerun. Status-specific evidence is validated before authority acceptance, unknown defects fail the job, and only explicit numerical execution failures become typed numerical_failure. Validation: 908 Vitest tests, 7 Playwright scenarios, TypeScript, ESLint, production build, docs governance, diff and 400-line gates. Real-browser click-cancel is honestly unclaimed because the genuine 2–4 trial job completes too quickly; transport lifecycle tests cover abort/termination/late events. Rust parity, prescribed production transport, full package persistence/import, and protected release remain open.