test(stammbaum): no two nodes overlap on the same row (#724)

O(n^2) sweep over canonical + synthetic: any two nodes sharing a y are at least
NODE_W + COL_GAP apart.

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

frontend/src/lib/person/genealogy/layout/buildLayout.test.ts

@@ -501,4 +501,42 @@ describe('buildLayout — ancestor centring invariant (#724)', () => {
 			forest.roots.forEach(walk);
 		}
 	});
+
+	it('no two nodes on the same row overlap (canonical + synthetic)', () => {
+		const cases: [string, PersonNodeDTO[], RelationshipDTO[]][] = [
+			['canonical', fixtureNodes, fixtureEdges],
+			[
+				'synthetic deep',
+				[
+					node('R', 'R', 0),
+					node('p1', 'p1', 1),
+					node('p2', 'p2', 1),
+					node('g1', 'g1', 2),
+					node('g2', 'g2', 2)
+				],
+				[
+					parentEdge('R', 'p1'),
+					parentEdge('R', 'p2'),
+					parentEdge('p1', 'g1'),
+					parentEdge('p1', 'g2')
+				]
+			]
+		];
+
+		for (const [label, nodes, edges] of cases) {
+			const layout = buildLayout(nodes, edges);
+			const entries = [...layout.positions.entries()];
+			for (let i = 0; i < entries.length; i++) {
+				for (let j = i + 1; j < entries.length; j++) {
+					const [, a] = entries[i];
+					const [, b] = entries[j];
+					if (a.y !== b.y) continue;
+					expect(
+						Math.abs(a.x - b.x),
+						`${label}: ${entries[i][0]} vs ${entries[j][0]} overlap on y=${a.y}`
+					).toBeGreaterThanOrEqual(NODE_W + COL_GAP);
+				}
+			}
+		}
+	});
 });