test(stammbaum): layout is deterministic under input reordering (#724)

Seeded Fisher-Yates permutation of nodes and edges yields byte-identical
positions — confirms every comparator ends in a stable id and nothing relies on
Map iteration order.

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

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

@@ -614,3 +614,40 @@ describe('buildLayout — ancestor centring invariant (#724)', () => {
 		expect(widest).toBeLessThanOrEqual(OLD_FULL_CANVAS / 4);
 	});
 });
+
+// Deterministic Fisher–Yates using a seeded mulberry32 PRNG (never Math.random,
+// which would make the determinism test itself non-reproducible).
+function seededShuffle<T>(input: T[], seed: number): T[] {
+	let s = seed >>> 0;
+	const rand = () => {
+		s = (s + 0x6d2b79f5) >>> 0;
+		let t = s;
+		t = Math.imul(t ^ (t >>> 15), t | 1);
+		t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+		return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+	};
+	const out = input.slice();
+	for (let i = out.length - 1; i > 0; i--) {
+		const j = Math.floor(rand() * (i + 1));
+		[out[i], out[j]] = [out[j], out[i]];
+	}
+	return out;
+}
+
+describe('buildLayout — determinism (#724)', () => {
+	const fixtureNodes = canonicalFixture.nodes as unknown as PersonNodeDTO[];
+	const fixtureEdges = canonicalFixture.edges as unknown as RelationshipDTO[];
+
+	it('produces identical positions regardless of node/edge input order', () => {
+		const base = buildLayout(fixtureNodes, fixtureEdges);
+		const shuffled = buildLayout(
+			seededShuffle(fixtureNodes, 1337),
+			seededShuffle(fixtureEdges, 4242)
+		);
+
+		expect(shuffled.positions.size).toBe(base.positions.size);
+		for (const [id, p] of base.positions) {
+			expect(shuffled.positions.get(id), `position for ${id} is order-independent`).toEqual(p);
+		}
+	});
+});