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

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

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

@@ -4,6 +4,7 @@ import {
 	parsePanZoomParams,
 	serializePanZoomParams,
 	screenDeltaToSvg,
+	zoomAtPoint,
 	DEFAULT_VIEW,
 	DEFAULT_ZOOM,
 	MIN_ZOOM,
@@ -91,3 +92,31 @@ describe('screenDeltaToSvg', () => {
 		expect(screenDeltaToSvg(10, 10, 1000, 800, 0, 0)).toEqual({ dx: 0, dy: 0 });
 	});
 });
+
+describe('zoomAtPoint', () => {
+	// The anchor is expressed as an offset (in SVG units) from the base viewBox
+	// centre. The fraction of the anchor across the viewBox must not change.
+	const anchorScreenFraction = (state: { x: number; z: number }, anchorOffsetX: number) => {
+		const baseW = 1000;
+		const w = baseW / state.z;
+		const centreOffset = anchorOffsetX - state.x; // anchor relative to viewBox centre
+		return centreOffset / w + 0.5;
+	};
+
+	it('keeps the canvas centre fixed when the anchor is the centre', () => {
+		const next = zoomAtPoint({ x: 0, y: 0, z: 1 }, 2, 0, 0);
+		expect(next).toEqual({ x: 0, y: 0, z: 2 });
+	});
+
+	it('keeps an off-centre anchor at the same screen position across a zoom-in', () => {
+		const before = { x: 0, y: 0, z: 1 };
+		const after = zoomAtPoint(before, 2, 100, 50);
+		expect(after.z).toBe(2);
+		expect(anchorScreenFraction(after, 100)).toBeCloseTo(anchorScreenFraction(before, 100), 10);
+	});
+
+	it('clamps the target zoom and makes no move when already at the bound', () => {
+		const next = zoomAtPoint({ x: 30, y: 10, z: MAX_ZOOM }, 99, 200, 200);
+		expect(next).toEqual({ x: 30, y: 10, z: MAX_ZOOM });
+	});
+});