Convert site from SPA hash routing to path-based static pages
Each page now has its own index.html with proper <title>, meta tags,
active nav highlighting, and shareable URLs. Old hash URLs (/#doctrine
etc.) redirect to the new paths. Markdown remains the content source
of truth via a build step (node build.js).
Pages: /, /start/, /doctrine/, /vocab/, /architecture/
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>