feat: give the chapters motion, diagrams, and retrieval practice

Three strands of work on the chapter surface, all reading from the same
constraint: this site ships no runtime dependencies, so every effect below
is native CSS or an .astro component.

Motion (src/styles/motion.css, DrawRule.astro). A scroll-driven layer of
reveals, hero parallax, section depth, and a hairline that paints itself
along its path, scrubbed by `animation-timeline: view()` — 0KB against
lottie-web's ~60KB gzipped on the main thread. Every scrubbed rule sits
inside `prefers-reduced-motion: no-preference` and `@supports`, so a
Firefox reader or an opted-out one gets the complete static page rather
than one with holes in it. Ranges key to `cover 40%`-`cover 75%` where the
motion is meant to be watched: `view()` ranges key to first visibility,
which on a 2600px page is long before anyone is reading the section.
`.agents/skills/motion/references/scroll-driven.md` records the technique
and the two ways `pathLength` normalisation was broken while building it.

Diagrams (StepFlow.astro, WorktreeMap.astro). The `.steps` stack on
/skills/, /models/, and /agents/ becomes a numbered flow with connectors,
and /agents/ grows the worktree map it was describing in prose — reusing
the `trees` collection rather than a second set of strings. The map's
static variant is gated one class deeper than full-guide's page styles, so
the interactive copy renders byte-identical. Connectors are pseudo-elements,
not SVG: a stretched path desynchronises its own dash pattern, and a
straight line does not need one. Mermaid was considered and rejected at
~1MB of runtime.

Retrieval practice (/rules/, /skills/). A "check yourself" section of
native `<details>` question/answer pairs plus a citation row, both bilingual
through the existing `data-copy` toggle, and both JavaScript-free.

Two audit blind spots surfaced and are closed rather than worked around:
`build.inlineStylesheets: 'never'`, because Astro inlined sheets under ~4kB
and audit-ui.mjs reads its colour and size baseline from dist/_astro/*.css;
and `--columns` declared in the grid components, because an element-level
custom property is not a declaration the audit can resolve.

legacy/styles/skills.css is renamed and imported for its side effect. Inside
a *page*, `?url` resolves to that page's own CSS chunk whatever file it
names, so the link pointed at the wrong asset and the sheet was emitted but
never loaded — the package preview had been rendering unstyled and
overflowing since the Astro cutover.

verify.mjs gains 5 assertions for the recall sections and their Portuguese
copy: 89 now, against the 84 baseline.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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