test(stammbaum): cover empty-graph and single-node layouts (#724)

Review follow-up (Sara/QA): the empty graph (fresh /stammbaum before data loads)
exercised the positions.size===0 viewBox fallback and the roots.length===0 early
return, both previously untested. Assert no NaN in the viewBox and MIN dimensions,
plus a single isolated node placed once at rank 0.

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

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

@@ -697,3 +697,27 @@ describe('buildLayout — termination + once-only on cyclic input (#724)', () =>
 		for (const id of [F, A, B, C]) expect(layout.positions.get(id)).toBeDefined();
 	});
 });
+
+describe('buildLayout — degenerate graphs (#724)', () => {
+	it('empty graph yields no positions and the MIN viewBox (no NaN)', () => {
+		// Runs on a fresh /stammbaum visit before any data loads — must not throw
+		// or emit NaN into the SVG viewBox.
+		const layout = buildLayout([], []);
+		expect(layout.positions.size).toBe(0);
+		expect(layout.crossLinks).toEqual([]);
+		expect(layout.generations.size).toBe(0);
+		for (const v of [layout.viewX, layout.viewY, layout.viewW, layout.viewH]) {
+			expect(Number.isFinite(v)).toBe(true);
+		}
+		// computeViewBox falls back to MIN dimensions when there is no content.
+		expect(layout.viewW).toBe(1200);
+		expect(layout.viewH).toBe(800);
+	});
+
+	it('single isolated node is placed once at rank 0', () => {
+		const layout = buildLayout([node(PARENT, 'Solo', 0)], []);
+		expect(layout.positions.size).toBe(1);
+		expect(layout.positions.get(PARENT)).toEqual({ x: 0, y: 0 });
+		expect(layout.crossLinks).toEqual([]);
+	});
+});