feat(stammbaum): animate fit-to-screen, snap under reduced motion (#692)

Fit-to-screen tweens to the default view over 300ms via animateView (eased,
lerpView-driven) and snaps instantly when prefers-reduced-motion is set
(US-PAN-004 AC2, NFR-A11Y-003).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/animateView.ts

@@ -0,0 +1,38 @@
+import { lerpView, type PanZoomState } from '$lib/person/genealogy/panZoom';
+
+/** Fit / recentre animation duration (US-PAN-004 AC2: ≤ 300 ms). */
+export const VIEW_ANIM_MS = 300;
+
+const easeOutCubic = (t: number) => 1 - Math.pow(1 - t, 3);
+
+/** Snapshot of the user's reduced-motion preference (non-reactive, browser-only). */
+export function prefersReducedMotion(): boolean {
+	return typeof window !== 'undefined' && typeof window.matchMedia === 'function'
+		? window.matchMedia('(prefers-reduced-motion: reduce)').matches
+		: false;
+}
+
+/**
+ * Tween the view from `from` to `to`, calling `onFrame` with each interpolated
+ * state. Honors reduced motion by snapping straight to the target (NFR-A11Y-003,
+ * REQ-PAN-005). Returns a cancel function.
+ */
+export function animateView(
+	from: PanZoomState,
+	to: PanZoomState,
+	onFrame: (state: PanZoomState) => void,
+	opts: { reducedMotion?: boolean; durationMs?: number } = {}
+): () => void {
+	if (opts.reducedMotion) {
+		onFrame(to);
+		return () => {};
+	}
+	const duration = opts.durationMs ?? VIEW_ANIM_MS;
+	const start = performance.now();
+	let raf = requestAnimationFrame(function step(now: number) {
+		const t = Math.min(1, (now - start) / duration);
+		onFrame(lerpView(from, to, easeOutCubic(t)));
+		if (t < 1) raf = requestAnimationFrame(step);
+	});
+	return () => cancelAnimationFrame(raf);
+}