test(stammbaum): add makeNode factory for birth-year ordering tests (#724)

The existing node() factory never sets birthYear, but the new sibling/branch
comparator (birthYear ASC NULLS LAST) needs it. Add makeNode(id, name,
{birthYear, generation}) alongside it; unblocks every ordering test.

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

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

@@ -20,6 +20,20 @@ function node(id: string, displayName: string, generation: number | null = null)
 		: { id, displayName, familyMember: true, generation };
 }
 
+// Richer factory than node(): lets ordering tests set birthYear (which the
+// sibling/branch comparator sorts on) and generation independently. node()
+// never sets birthYear, so every birth-year ordering assertion needs this.
+function makeNode(
+	id: string,
+	displayName: string,
+	opts: { birthYear?: number; generation?: number } = {}
+): PersonNodeDTO {
+	const n: PersonNodeDTO = { id, displayName, familyMember: true };
+	if (opts.birthYear != null) n.birthYear = opts.birthYear;
+	if (opts.generation != null) n.generation = opts.generation;
+	return n;
+}
+
 function parentEdge(parentId: string, childId: string, id = parentId + childId): RelationshipDTO {
 	return {
 		id,
@@ -48,6 +62,19 @@ function yOf(layout: ReturnType<typeof buildLayout>, id: string): number {
 	return p.y;
 }
 
+describe('makeNode factory', () => {
+	it('sets birthYear and generation only when provided', () => {
+		expect(makeNode('a', 'A')).toEqual({ id: 'a', displayName: 'A', familyMember: true });
+		expect(makeNode('b', 'B', { birthYear: 1900, generation: 2 })).toEqual({
+			id: 'b',
+			displayName: 'B',
+			familyMember: true,
+			birthYear: 1900,
+			generation: 2
+		});
+	});
+});
+
 describe('buildLayout — generation seeding (#689)', () => {
 	it('Herbert Cram regression: two parented G=3 spouses share the same row', () => {
 		// Both Herbert (G 3) and Clara (G 3) are parented children of their respective