test(stammbaum): cross-level marriage records a distinct cross-link (#724)

When the two spouses' parents sit at different structural levels, the
structural owner keeps its hierarchy edge and the other parent->spouse edge is
recorded in layout.crossLinks (rendered with a distinct dash). The couple still
sits exactly adjacent in the owner's run and B keeps a real position.

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

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

@@ -364,3 +364,44 @@ describe('buildLayout — multi-spouse ordering (#361)', () => {
 		}
 	});
 });
+
+describe('buildLayout — cross-level marriage fallback (#724)', () => {
+	// A bond is cross-level when the two spouses' parents sit at different
+	// structural levels. Adjacency cannot keep both connectors short, so the
+	// structural owner keeps its hierarchy edge and the other parent → spouse
+	// edge becomes a distinct cross-link.
+	const GP = '00000000-0000-0000-0000-0000000000e1'; // G0, deep branch ancestor
+	const P = '00000000-0000-0000-0000-0000000000e2'; // G1, child of GP
+	const A = '00000000-0000-0000-0000-0000000000e3'; // G2, child of P (nested deep)
+	const R = '00000000-0000-0000-0000-0000000000e4'; // G1 root
+	const B = '00000000-0000-0000-0000-0000000000e5'; // G2, child of R
+
+	it('records the displaced parent edge as a cross-link and keeps the couple adjacent', () => {
+		const layout = buildLayout(
+			[
+				node(GP, 'GP', 0),
+				node(P, 'P', 1),
+				{ ...node(A, 'A', 2), birthYear: 1900 }, // earlier birth → A is structural owner
+				node(R, 'R', 1),
+				{ ...node(B, 'B', 2), birthYear: 1910 }
+			],
+			[
+				parentEdge(GP, P, 'g1'),
+				parentEdge(P, A, 'g2'),
+				parentEdge(R, B, 'g3'),
+				spouseEdge(A, B, 'sp')
+			]
+		);
+
+		// A owns; B is absorbed into A's run → couple is exactly adjacent.
+		const posA = layout.positions.get(A)!;
+		const posB = layout.positions.get(B)!;
+		expect(posA.y).toBe(posB.y);
+		expect(Math.abs(posA.x - posB.x)).toBe(NODE_W + COL_GAP);
+
+		// R → B is cross-level: it surfaces as a distinct cross-link, and the
+		// geometry still lands on B's real position (carried redundantly).
+		expect(layout.crossLinks).toEqual([{ parentId: R, childId: B }]);
+		expect(layout.positions.get(B)).toBeDefined();
+	});
+});