feat(skills): site-motion, site-level scroll and transition choreography

Personal skill distilling the MengTo motion pack invariants (LRN-141):
gates first (reduced motion renders final states, content visible without
JS, compositor-only, offscreen pause), one smooth-scroll engine with the
Lenis/ScrollTrigger sync, Astro ClientRouter lifecycle, numbered recipes
(reveal, scrub, sticky stack, video and image scrub, TreeWalker split,
progressive blur, marquee, WebGL budgets), upstream pitfalls, checklist.
Routed into the Build UI chain of CLAUDE.global.md and lib/design-gate.md.

lib/design-gate.md

@@ -47,6 +47,9 @@ browser/plan/shotgun tooling and graphify for convenience; those never trip the
 gate. Motion (`design-motion-principles`) and static-HTML (`design-html`) are
 already in the core set — checked regardless; their CLAUDE.md "+motion /
 +static" notes say which tool you'll lean on, not a separate activation step.
+`site-motion` (personal skill, site-level scroll/page choreography) rides the
+same Build chain but isn't on the GATE-BLOCK list: it ships with the repo,
+nothing to install or verify.
 
 ### 2. State — run the deterministic check