Implement pre-launch product requirements

[damonko1]

Summary

Implements the approved pre-launch PRD while preserving the simplified House-first product hierarchy. This remains isolated from main until release review is complete.

Product changes

• strengthens the in-product scenario/not-a-forecast boundary
• adds SVG and PNG scenario-card export
• returns malformed links safely to the House launch default
• enforces 44px mobile touch targets
• keeps House and Senate primary and presidential replay under History
• retains expert capabilities behind progressive disclosure

Release infrastructure

• adds a launch-contract validator to CI
• adds a support, severity, monitoring, deployment, and rollback runbook
• adds a requirement-by-requirement release evidence matrix
• updates all remaining product identity and usage documentation
• clears production and development dependency advisories

Verification

• ESLint and TypeScript pass
• 37 tests pass
• state, House-map, and legislative-data validators pass
• launch-contract validator passes
• standard and GitHub Pages base-path builds pass
• full npm audit reports zero vulnerabilities
• Chromium desktop/mobile, keyboard, themes, malformed URLs, and SVG/PNG downloads verified
• clean-checkout branch quality gate passed

Deliberately pending

• native Safari confirmation on a supported environment
• moderated first-time-user tasks
• merge, production deployment, and public smoke test

See docs/prelaunch-product-requirements.md, docs/prelaunch-release-evidence.md, and docs/release-operations.md.