feat: ship v0.5 auditable demo workbench

[Kristariss]

Summary

• add a live six-step Hefei–NIO negotiation replay with common-random-number fiscal counterfactuals
• model separately governed joint investment funds and publish fiscal-space sensitivity results
• integrate an experiment report center with strategy, ablation, seed, and failure drill-down
• align product docs, roadmap, changelog, one-command demo, and v0.5 release metadata

Validation

• make verify
• 62 Python tests passed; Ruff passed
• Web ESLint, TypeScript, rendered HTML test, and production build passed
• agent-browser: home, live demo, internal chain, counterfactual, report center, failed seed, and ablation views passed with no console errors
• tracked-file secret scan passed; .env, .insidegov, logs, PDFs, and build outputs remain ignored

Research boundary

The Hefei–NIO scenario is a source-backed mechanism calibration, not a causal estimate or one-to-one reconstruction of historical actors.