perf(web): statically compile chapter full-text rollups during build

[williamzujkowski]

Description

Statically pre-renders all section contents within chapter rollups at build time using Astro SSG, replacing client-side sequential fetch() and DOMParser waterfalls for large chapters (>50 sections).

Key Improvements

• Zero Client Network Waterfalls: When reading full chapter text, all articles are already compiled into the static document, eliminating 50+ network requests on mobile/slow connections.
• Instant Toggling: Toggling between 'Sections in this chapter' and 'Read full chapter' is instantaneous (0ms) and entirely client-side without spinners or parsing delays.
• Pagefind Search Indexing: Static compilation ensures Pagefind search indexes full chapter texts alongside section-level pages.
• Deep-linking & Jump Anchors: Retains #section-{id} anchor linking, auto-expanding the chapter text when loaded with a section hash.
• Cross-Reference Auto-linking: Automatically links section citations (e.g. section 102) across all rendered chapter articles.

Closes #279