refactor(stammbaum): TestNode type alias drops generation cast (#361)

Introduces a local `type TestNode = { id: string; generation: number | null }`
so the three AC3 test fixtures can write `generation: null` directly,
without the awkward `as number | null` cast next to the literal `generation:
2`. Sara cycle-3 cosmetic; same predicate, cleaner reading.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/__fixtures__/validateFixture.test.ts

@@ -7,6 +7,11 @@ import canonicalFixture from './stammbaum.json';
 // buildLayout.test.ts relies on. Adding or removing a gate without updating
 // these tests is the failure we want to catch.
 
+// Lets `generation: null` typecheck without an inline `as number | null`
+// cast — the AC3 predicate cares specifically about the unseeded-node
+// branch, so the test fixtures need to express it directly.
+type TestNode = { id: string; generation: number | null };
+
 function networkWithNodes(count: number) {
 	return {
 		nodes: Array.from({ length: count }, (_, i) => ({ id: `n${i}`, generation: i % 6 })),
@@ -87,9 +92,9 @@ describe('findAc3Candidates — AC3 revisit-trigger predicate (#361, Elicit cycl
 	});
 
 	it('flags_an_unseeded_person_whose_seeded_parent_is_in_graph_and_who_has_a_spouse', () => {
-		const nodes = [
+		const nodes: TestNode[] = [
 			{ id: 'parent', generation: 2 },
-			{ id: 'child', generation: null as number | null },
+			{ id: 'child', generation: null },
 			{ id: 'spouse', generation: 3 }
 		];
 		const edges = [parentEdge('parent', 'child'), spouseEdge('child', 'spouse')];
@@ -100,9 +105,9 @@ describe('findAc3Candidates — AC3 revisit-trigger predicate (#361, Elicit cycl
 		// Unseeded + seeded parent in graph, but no SPOUSE_OF — not AC3 (the
 		// layout branch that hurts is the *loose spouse* branch). The capture
 		// warn must not fire here.
-		const nodes = [
+		const nodes: TestNode[] = [
 			{ id: 'parent', generation: 2 },
-			{ id: 'child', generation: null as number | null }
+			{ id: 'child', generation: null }
 		];
 		const edges = [parentEdge('parent', 'child')];
 		expect(findAc3Candidates({ nodes, edges })).toEqual([]);
@@ -112,9 +117,9 @@ describe('findAc3Candidates — AC3 revisit-trigger predicate (#361, Elicit cycl
 		// "Parents in graph" in the AC3 sense means at least one *seeded*
 		// parent. If both parent and child are unseeded the heuristic
 		// fallback already handles the case without AC3 ever firing.
-		const nodes = [
-			{ id: 'parent', generation: null as number | null },
-			{ id: 'child', generation: null as number | null },
+		const nodes: TestNode[] = [
+			{ id: 'parent', generation: null },
+			{ id: 'child', generation: null },
 			{ id: 'spouse', generation: 3 }
 		];
 		const edges = [parentEdge('parent', 'child'), spouseEdge('child', 'spouse')];