Install the motion preference at app root and reflect it onto
<html data-motion>, driving the reduced-motion safety net in
motion-tokens.css. Unlike the density reflector, this one removes
the attribute for 'auto' so the OS prefers-reduced-motion
media query stays in charge, and sets it for 'reduced' / 'full'. The
reflector is an effect (not afterNextRender) so it re-runs on every
runtime injectMotion().set(...); the DOM write is wrapped in
untracked() per the signal-architecture rules.