test(stammbaum): assert AC8 recentre via viewBox, not replaceState (#703)

The desktop AC8 test flaked in CI: it asserted replaceState was never
called after a tap, but the mount-time URL mirror fired late with the
unchanged default view (cx=0&cy=0&z=1), tripping the assertion. Assert on
the rendered viewBox instead — a pure function of the view state — so a
recentre shows as a shifted origin and a desktop tap leaves it identical,
with no dependence on the noisy mirror-effect timing.

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

frontend/src/routes/stammbaum/page.svelte.test.ts

@@ -157,6 +157,15 @@ describe('stammbaum page', () => {
 	// desktop side panel is a flex sibling that never overlaps the canvas, so no
 	// centring should fire there. These tests prove the matchMedia gate around
 	// selectPerson, not just the recentreAbove geometry (covered in panZoom.test).
+	// We assert on the rendered viewBox — a pure function of the view state — so a
+	// recentre is observed as a shifted origin, with no dependence on the noisy
+	// mount-time URL-mirror timing.
+	function svgViewBox(): string {
+		const svg = document.querySelector('svg[aria-label="Stammbaum"]');
+		if (!svg) throw new Error('No Stammbaum svg rendered');
+		return svg.getAttribute('viewBox') ?? '';
+	}
+
 	it('recentres the tapped person when matchMedia reports mobile (#703 AC8)', async () => {
 		mockMatchMedia(true);
 		mockPage.url = new URL('http://localhost/stammbaum');
@@ -164,15 +173,17 @@ describe('stammbaum page', () => {
 		render(Stammbaum, {
 			props: { data: { nodes: familyNodes, edges: [], initialView: DEFAULT_VIEW } }
 		});
-		// Let the mount-time URL mirror settle, then isolate the tap's effect.
-		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
-		replaceState.mockClear();
+		const before = await vi.waitFor(() => {
+			const vb = svgViewBox();
+			expect(vb).toBeTruthy();
+			return vb;
+		});
 
 		await page.getByRole('button', { name: 'Anna Schmidt' }).click();
 
-		// The mobile tap recentres the canvas → the view changes → the ?cx&cy&z
-		// mirror effect re-fires. (Desktop, below, leaves the view untouched.)
-		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
+		// The mobile tap recentres the canvas, lifting the anchor above the sheet,
+		// so the viewBox origin shifts.
+		await vi.waitFor(() => expect(svgViewBox()).not.toBe(before));
 	});
 
 	it('does not recentre on tap when matchMedia reports desktop (#703 AC8)', async () => {
@@ -182,14 +193,17 @@ describe('stammbaum page', () => {
 		render(Stammbaum, {
 			props: { data: { nodes: familyNodes, edges: [], initialView: DEFAULT_VIEW } }
 		});
-		await vi.waitFor(() => expect(replaceState).toHaveBeenCalled());
-		replaceState.mockClear();
+		const before = await vi.waitFor(() => {
+			const vb = svgViewBox();
+			expect(vb).toBeTruthy();
+			return vb;
+		});
 
 		await page.getByRole('button', { name: 'Anna Schmidt' }).click();
 
 		// The tap registers — the desktop side panel opens — but no recentre fires,
-		// so the view never changes and the mirror effect stays silent.
+		// so the viewBox is unchanged.
 		await expect.element(page.getByRole('complementary')).toBeVisible();
-		expect(replaceState).not.toHaveBeenCalled();
+		expect(svgViewBox()).toBe(before);
 	});
 });