Shell's page content now remounts on every navigation and plays a
scale+fade pop-in entrance. The active nav link's underline is now a
single shared element that slides to its new position instead of
snapping, re-colouring to a per-tab accent — a discrete cycle through
the existing base16 chromatic slots, not a continuous hue rotation.
Both are gated on the data-motion/prefers-reduced-motion plumbing
lib/motion-apply.ts already had wired and waiting for a first real
consumer.
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