Reduced motion: honor prefers-reduced-motion for video + logo, add hero pause control (#1294)

[jonfroehlich]

Closes #1294.

Makes the three remaining motion sources honor the OS-level prefers-reduced-motion preference, and adds a WCAG 2.2.2 (Pause, Stop, Hide, Level A) pause mechanism for the auto-moving hero. No in-page motion toggle (that was rejected in #1196).

Approach — progressive enhancement

CSS can't stop <video> playback or a <canvas> loop, so JS is genuinely required for those. Guiding principle: the no-JS HTML baseline delivers the content; JS only removes motion for people who asked for less.

• The hero <video> keeps autoplay muted loop — JS-disabled users get the video exactly as today (no regression). JS pauses it under reduced motion / via the pause control; it never enables it.
• Rejected the <source media="(prefers-reduced-motion: no-preference)"> trick — Chrome/Safari dropped media on <video>/<source> (Firefox-only now).

Three surfaces tied together by one shared helper and one pause control.

Changes

• js/reduced-motion.js (new): shared window.MakeLab.prefersReducedMotion(), queried live (an OS toggle mid-session is respected without reload). Loaded before carousel.js; the makelab-logo.js module reads it too.
• Hero video (base.html + carousel.js): keep autoplay; add a poster from the banner's existing image when set (no model change); carousel.js pauses the active video under reduced motion so the poster shows.
• Logo canvas (makelab-logo.js): under reduced motion, pin the morph to its assembled end state (lerp = 1), skip the scroll handler (keep ResizeObserver for sizing), and drop "Animated" from the canvas label.
• WCAG 2.2.2 (base.html + carousel_fade.css + carousel.js): one visible, keyboard-operable Pause/Play toggle governing both the auto-advance and the looping video. Real <button>, dynamic aria-label/icon, ≥44×44 target, contrast chip, focus ring. Ships hidden so no-JS users never see a dead control; carousel.js reveals it only when motion exists.

Testing

• New regression tests (test_hero_motion.py): hero video keeps autoplay, gets a poster only when the banner has an image, and the pause control renders. 4/4 pass.
• Full suite: 412 pass / 8 skipped, no regressions.
• Pa11y: neither the pause button nor the video appears in the output → no new violations (the failing baseline is the site's pre-existing color-contrast debt, unrelated).
• Manual: verified on macOS toggling Reduce motion on/off in Accessibility settings — video, logo, and carousel all respond live; pause button keyboard-operable.

Screenshots

Refs #1196, #1288, #1293.

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