fix(dict): flash relationship after scroll settles

[damusix]

Summary

• wait for the Dictionary browse-lens scroll position to settle before flashing a relationship target
• require the target to be visible and cancel stale flashes on newer navigation
• honor reduced-motion preferences with instant scrolling
• update the visual regression and spotlight design/spec contracts

Verification

• bun run build:bundle
• bun test/visual/test-dd-spotlight-grid.ts — CP5 passed: no early flash, 100 ms scroll quiet period, target visible at flash start, class removed on animation end
• visually reviewed tmp/dd-spotlight-grid/29-cp5-after-chip-click.png

The broader visual script passed the relevant CP5 checkpoint and continued through CP14, then stopped at the unrelated CP15 synthetic-model server startup check.