test(stammbaum): cyclic input fails closed — finite layout, one position per node (#724)

An A<->B parent cycle and a founder reaching a re-entrant 3-cycle both return a
finite layout (no frozen $derived) with every node placed exactly once.

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

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

@@ -651,3 +651,49 @@ describe('buildLayout — determinism (#724)', () => {
 		}
 	});
 });
+
+describe('buildLayout — termination + once-only on cyclic input (#724)', () => {
+	// buildLayout runs inside a $derived, so non-termination = a frozen tab.
+	// User-entered data can form parent cycles; the layout must fail closed —
+	// produce *a* finite layout, every node placed exactly once.
+	const A = '00000000-0000-0000-0000-0000000000a1';
+	const B = '00000000-0000-0000-0000-0000000000a2';
+	const C = '00000000-0000-0000-0000-0000000000a3';
+
+	it('A↔B parent cycle returns a finite layout with one position per node', () => {
+		let layout!: ReturnType<typeof buildLayout>;
+		expect(() => {
+			layout = buildLayout(
+				[node(A, 'A'), node(B, 'B')],
+				[parentEdge(A, B, 'ab'), parentEdge(B, A, 'ba')]
+			);
+		}).not.toThrow();
+
+		expect(layout.positions.size).toBe(2);
+		expect(layout.positions.get(A)).toBeDefined();
+		expect(layout.positions.get(B)).toBeDefined();
+	});
+
+	it('two founders both reaching a re-entrant 3-cycle terminate with every node placed', () => {
+		// F is a founder parenting A; A→B→C→A is a parent cycle, and C also
+		// re-enters via F. The visited/once-only guarantees keep it finite.
+		const F = '00000000-0000-0000-0000-0000000000a0';
+		let layout!: ReturnType<typeof buildLayout>;
+		expect(() => {
+			layout = buildLayout(
+				[node(F, 'F'), node(A, 'A'), node(B, 'B'), node(C, 'C')],
+				[
+					parentEdge(F, A, 'fa'),
+					parentEdge(A, B, 'ab'),
+					parentEdge(B, C, 'bc'),
+					parentEdge(C, A, 'ca')
+				]
+			);
+		}).not.toThrow();
+
+		// Every node is placed exactly once (Map keys are unique by construction;
+		// the count equality proves none were dropped or duplicated).
+		expect(layout.positions.size).toBe(4);
+		for (const id of [F, A, B, C]) expect(layout.positions.get(id)).toBeDefined();
+	});
+});