Load page assets only when needed

[mre]

Summary

• extract 55 KB of shared CSS from the 114 KB base template into a cacheable static stylesheet, reducing dynamic base HTML to 34 KB
• self-host one 47 KB Inter variable WOFF2 for the used 400-700 range; remove Google Fonts entirely
• bundle and self-host CodeMirror, Lezer, Rust highlighting, and Vim mode as split, shared local chunks; remove the global esm.sh import map and all direct esm.sh imports
• add a pinned npm lockfile and CI drift check for generated bundles; production serves checked-in assets and does not require Node
• load quiz.js only for the quiz chapter
• fetch canvas-confetti only after a successful run requests the celebration
• defer the read-only CodeMirror solution viewer until a solution is revealed
• skip loading the Rust prose parser on pages without prose code blocks

Observed impact

• base.html: 114,158 bytes -> 34,341 bytes
• initial page loads make no requests to Google Fonts, fonts.gstatic.com, or esm.sh
• Inter is served as one local variable font covering 400-700
• quiz.js is absent from dashboard, normal chapters, and tour
• canvas-confetti is absent from initial page requests
• readonly editor dependencies are absent before solution reveal

Validation

• cargo fmt --all --check
• cargo clippy --locked --lib --bins -- -D warnings
• cargo test --locked --lib --bins
• ./scripts/check-examples.sh
• REQUIRE_COMPLETE=1 ./scripts/check-solutions.sh (75/75)
• npm audit --omit=dev (0 vulnerabilities)
• rebuilt bundles and syntax-checked every generated JS file
• browser smoke test: dashboard, normal exercise, tour, cheatsheet, settings, admin, editable editors, and lazy read-only solution viewer
• browser network test: all local font/dist requests 200; zero Google Fonts or esm.sh requests