test(stammbaum): same-level intra-family bond renders solid, not a cross-link (#724)

Extends the existing adjacency contract: the couple is exactly adjacent in the
run AND, because both parents are roots (same structural level), the displaced
parent edge stays solid — layout.crossLinks is empty for this case.

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

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

@@ -306,6 +306,11 @@ describe('buildLayout — multi-spouse ordering (#361)', () => {
 			if (p.y !== posA2.y) continue;
 			expect(p.x <= minX || p.x >= maxX).toBe(true);
 		}
+
+		// Same-level bond (both parents are roots): the displaced parent edge
+		// can be ordered adjacent, so it stays a solid connector — NOT a dashed
+		// cross-link. The cross-link set is therefore empty for this case.
+		expect(layout.crossLinks).toEqual([]);
 	});
 
 	it('canonical_fixture_multi_spouse_falls_through_to_displayName_when_no_fromYear', () => {