test(stammbaum): a bloodline occupies one contiguous band (#724)

No node outside a root's structural subtree may intrude into that bloodline's
[minX, maxX] horizontal span — the contiguity guarantee that fixes the smeared
bloodline symptom.

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

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

@@ -539,4 +539,46 @@ describe('buildLayout — ancestor centring invariant (#724)', () => {
 			}
 		}
 	});
+
+	it('a bloodline occupies one contiguous band — no foreign node interleaved', () => {
+		// Two roots, each its own bloodline. The first fans out two generations
+		// deep; the contour pack must keep it as one band with nothing from the
+		// other bloodline wedged inside its horizontal span.
+		const nodes = [
+			node('R1', 'R1', 0),
+			node('a', 'a', 1),
+			node('b', 'b', 1),
+			node('a1', 'a1', 2),
+			node('R2', 'R2', 0),
+			node('c', 'c', 1)
+		];
+		const edges = [
+			parentEdge('R1', 'a'),
+			parentEdge('R1', 'b'),
+			parentEdge('a', 'a1'),
+			parentEdge('R2', 'c')
+		];
+		const layout = buildLayout(nodes, edges);
+		const forest = buildFamilyForest(nodes, edges);
+
+		// Collect the R1 bloodline: every member of its unit subtree.
+		const r1 = forest.roots.find((u) => u.id === 'R1')!;
+		const bloodline = new Set<string>();
+		const collect = (u: Unit) => {
+			u.members.forEach((m) => bloodline.add(m));
+			u.children.forEach(collect);
+		};
+		collect(r1);
+
+		const bandXs = [...bloodline].map((id) => layout.positions.get(id)!.x);
+		const minX = Math.min(...bandXs);
+		const maxX = Math.max(...bandXs) + NODE_W;
+
+		// No node outside the bloodline may intrude into its [minX, maxX] band.
+		for (const [id, p] of layout.positions) {
+			if (bloodline.has(id)) continue;
+			const intrudes = p.x < maxX && p.x + NODE_W > minX;
+			expect(intrudes, `foreign node ${id} interleaved into the R1 band`).toBe(false);
+		}
+	});
 });