feat: improve adoption readiness and shareable demo

[krkarma777]

Summary

• make the v1.2 demo shareable across all five diff modes and options
• restore URL state safely, report diff statistics, and guard oversized automatic comparisons
• add adoption-focused documentation, contribution guidance, and GitHub templates
• align package metadata, JSDoc, social metadata, and the generated social card
• stop tracking generated coverage artifacts

Verification

• npm run typecheck
• npm test — 83/83 passed
• npm run build
• Added or updated tests for behavior changes
• Ran npm run perf:smoke; core diff behavior is unchanged, so comparative npm run bench is not required
• Updated CHANGELOG.md

Evidence

• Coverage: 99.76% statements, 96.89% branches, 100% functions
• Browser bundle: 8,876 bytes raw / 3,932 bytes gzip
• npm pack dry run: expected 11-file allowlist only
• Chromium file URL checks covered all modes, option wiring, sharing/restoration, errors, responsive layouts, and the 16,000/16,001 automatic-render boundary
• Final independent review: 0 Critical, 0 Important findings