The one render chain: remark-gfm, remark-directive (mapped-tag escape
hatch for future embed islands), rehype-figure, rehype-pretty-code with
vesper highlighting at build time. Shipped pages are flat HTML with
inline token colors and zero client JS.

- lib/markdown.ts mints all /blog HTML; lib/blog.js binds the section
- app/blog + app/blog/[slug] on the App Router (SSG, generateMetadata);
  app/post/[id] and lib/posts.js removed with the route
- content/posts moved to content/blog; the 2021 entries are draft:true,
  hello-world-again is the one published post (engine fixture: figure
  captioning, shiki, GFM table) with its edge.png image
- yarn post / yarn post:image scaffold posts and stage images
- e2e smoke routes follow the rename