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/data/constellation.ts

@@ -1,3 +1,5 @@
+// Module-level cache: buildConstellationData() is expensive (D3 graph construction).
+// 5 consumers import from here instead of calling the builder independently.
 import type { ConstellationLink, ConstellationNode, RoleSkillMapping } from '@/types/pmr'
 import { buildConstellationData } from '@/data/timeline'
 

src/data/tags.ts

@@ -1,3 +1,5 @@
+// Derives sidebar tags from timeline skills with color assignment.
+// Separated from Sidebar.tsx to keep data derivation out of UI components.
 import type { Tag } from '@/types/pmr'
 import { getTopTimelineSkills } from '@/data/timeline'