docs(e2e): clear the stale known-issue markers, root cause found

checkout-request-shape.spec.ts and checkout-idempotent-click.spec.ts were
flagged known-failing pending investigation; dda0a3d found and fixed the
actual cause (circular DI in apiHeadersInterceptor). Update the comments
and README so they no longer point at an unresolved mystery.

e2e/checkout-idempotent-click.spec.ts

@@ -20,9 +20,8 @@ test('double-clicking checkout sends exactly one checkout-session request', asyn
     window.localStorage.setItem('marketplace_cart', JSON.stringify([item]));
   }, FAKE_ITEM);
 
-  // KNOWN ISSUE, NOT RESOLVED (2026-08-21) - see checkout-request-shape.spec.ts's
-  // fakeCustomerSession comment. This test currently fails the same way:
-  // the session check never fires despite the cookie being present.
+  // Root-caused and fixed 2026-08-21 - see checkout-request-shape.spec.ts's
+  // fakeCustomerSession comment and api-headers.interceptor.ts.
   await context.addCookies([{ name: 'webSessionID', value: 'e2e-fake-session', url: 'http://localhost:4200' }]);
   await page.route('**/users/sessions/**', route =>
     route.fulfill({