feat(stammbaum): centroid-anchored zoom (zoomAtPoint) (#692)

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

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

@@ -81,3 +81,28 @@ export function screenDeltaToSvg(
 		dy: elPxH > 0 ? dyPx * (viewBoxH / elPxH) : 0
 	};
 }
+
+/**
+ * Zoom to `newZoom` while keeping a given anchor point fixed on screen
+ * (pinch-centroid zoom — US-PAN-002 AC1 / US-PAN-003 AC1).
+ *
+ * `anchorX`/`anchorY` are the anchor point expressed as an offset, in SVG units,
+ * from the base viewBox centre. Because the viewBox width scales as `1/z`, the
+ * ratio of old-to-new width is exactly `z / newZoom` independent of the base
+ * size, so the new pan offset that preserves the anchor's screen fraction is
+ * `anchor - (anchor - pan) * (z / newZoom)`.
+ */
+export function zoomAtPoint(
+	state: PanZoomState,
+	newZoom: number,
+	anchorX: number,
+	anchorY: number
+): PanZoomState {
+	const z = clampZoom(newZoom);
+	const ratio = state.z / z;
+	return {
+		x: anchorX - (anchorX - state.x) * ratio,
+		y: anchorY - (anchorY - state.y) * ratio,
+		z
+	};
+}