feat(stammbaum): recentre-on-node with legible auto-zoom (#692)

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

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

@@ -5,8 +5,10 @@ import {
 	serializePanZoomParams,
 	screenDeltaToSvg,
 	zoomAtPoint,
+	recentreOn,
 	DEFAULT_VIEW,
 	DEFAULT_ZOOM,
+	LEGIBLE_ZOOM,
 	MIN_ZOOM,
 	MAX_ZOOM
 } from './panZoom';
@@ -120,3 +122,26 @@ describe('zoomAtPoint', () => {
 		expect(next).toEqual({ x: 30, y: 10, z: MAX_ZOOM });
 	});
 });
+
+describe('recentreOn', () => {
+	const node = { x: 300, y: 200 };
+	const base = { x: 100, y: 100 };
+
+	it('pans so the node sits at the viewBox centre, keeping the current zoom', () => {
+		expect(recentreOn(node, base, { x: 0, y: 0, z: 1 }, false)).toEqual({ x: 200, y: 100, z: 1 });
+	});
+
+	it('auto-zooms up to LEGIBLE_ZOOM when zoomed out (OQ-005)', () => {
+		const next = recentreOn(node, base, { x: 0, y: 0, z: 0.4 }, true);
+		expect(next.z).toBe(LEGIBLE_ZOOM);
+		expect({ x: next.x, y: next.y }).toEqual({ x: 200, y: 100 });
+	});
+
+	it('does not reduce an already-legible zoom when auto-zooming', () => {
+		expect(recentreOn(node, base, { x: 0, y: 0, z: 2 }, true).z).toBe(2);
+	});
+
+	it('leaves zoom untouched when auto-zoom is off', () => {
+		expect(recentreOn(node, base, { x: 0, y: 0, z: 0.4 }, false).z).toBe(0.4);
+	});
+});