fix(compare): stale basket fetch no longer blanks a freshly opened comparison

Opening a compare link while localStorage held a different basket raced a
fetch for the OLD school set against the URL's SSR data; the stale
response replaced comparisonData, so no active school had data and every
section (including the trends chart) vanished until a hard refresh.
Responses from superseded effect runs are now dropped, successful ones
merge instead of replace, and the URL-seed effect re-runs when a
client-side navigation changes ?urns=.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

e2e/tests/journeys.spec.ts

@@ -239,6 +239,31 @@ test('comparing two secondary schools renders the secondary sections', async ({
   await expect(page.getByText(/No primary schools in your comparison/)).toHaveCount(0);
 });
 
+test('opening a different compare link after a previous comparison still renders', async ({ page }) => {
+  // Regression: the first visit stores a basket in localStorage; opening a
+  // link for a DIFFERENT school set then raced a stale fetch for the stored
+  // basket against the new SSR data, blanking every section (including the
+  // trends chart) until a hard refresh.
+  const [s0, s1] = await twoSecondaryUrns(page);
+  const [p0, p1] = await twoPrimaryUrns(page);
+
+  await page.goto(`/compare?urns=${s0},${s1}`);
+  await expect(page.getByRole('heading', { name: 'At a glance' }).first()).toBeVisible({
+    timeout: 15_000,
+  });
+
+  await page.goto(`/compare?urns=${p0},${p1}`);
+  await expect(page.getByRole('heading', { name: 'At a glance' }).first()).toBeVisible({
+    timeout: 15_000,
+  });
+  // Give any straggling stale response time to land, then confirm the new
+  // comparison is still on screen.
+  await page.waitForTimeout(1500);
+  await expect(page.getByRole('heading', { name: 'At a glance' }).first()).toBeVisible();
+  await expect(page.getByRole('heading', { name: 'Explore trends' }).first()).toBeVisible();
+  await expect(page.locator(`a[href*="${p0}"]`).first()).toBeVisible();
+});
+
 test('compare chart on mobile shows school chips with tap-to-focus', async ({ page }) => {
   await page.setViewportSize({ width: 390, height: 844 });