refactor: extract hexToRgba and prefersReducedMotion to shared utils

Move hexToRgba() (3 identical copies) and prefersReducedMotion (5 module-level
copies) to src/lib/utils.ts. Re-export prefersReducedMotion from
constellation/constants.ts to preserve existing importers. Add clarifying
comments to constellation.ts and tags.ts re-export layers (Phase 1.4).

src/components/TimelineInterventionsSubsection.tsx

@@ -5,15 +5,7 @@ import { useDetailPanel } from '@/contexts/DetailPanelContext'
 import { timelineEntities, timelineConsultations } from '@/data/timeline'
 import { getExperienceEducationUICopy } from '@/lib/profile-content'
 import type { TimelineEntity } from '@/types/pmr'
-
-const prefersReducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches
-
-function hexToRgba(hex: string, opacity: number): string {
-  const r = parseInt(hex.slice(1, 3), 16)
-  const g = parseInt(hex.slice(3, 5), 16)
-  const b = parseInt(hex.slice(5, 7), 16)
-  return `rgba(${r},${g},${b},${opacity})`
-}
+import { hexToRgba, prefersReducedMotion } from '@/lib/utils'
 
 interface TimelineInterventionItemProps {
   entity: TimelineEntity