Network helpers, docs hygiene, Outline sync notes

docs/NETWORK.md

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+# Network helpers (page traffic)
+
+`ApiClient` *makes* HTTP calls from the test process. These helpers watch what the
+**browser page** itself does during UI flows — spy/mock specific calls, and fail
+tests that silently collect 4xx/5xx in the background.
+
+## Spy / mock — `interceptNetworkCall`
+
+Set up **before** the action that triggers the request:
+
+```ts
+import { interceptNetworkCall } from '@levkin/playkit';
+
+const users = interceptNetworkCall({
+  page,
+  method: 'GET',
+  url: '**/api/users',
+});
+
+await page.goto('/users');
+const { status, responseJson } = await users;
+expect(status).toBe(200);
+```
+
+Stub a response:
+
+```ts
+const users = interceptNetworkCall({
+  page,
+  url: '**/api/users',
+  fulfillResponse: {
+    status: 200,
+    body: [{ id: 1, name: 'e2e' }],
+  },
+});
+
+await page.goto('/users');
+await users;
+await expect(page.getByText('e2e')).toBeVisible();
+```
+
+## Background errors — `startNetworkErrorMonitor`
+
+Catches silent backend failures while the UI still looks fine:
+
+```ts
+import { startNetworkErrorMonitor } from '@levkin/playkit';
+
+test('dashboard stays clean', async ({ page }) => {
+  const net = startNetworkErrorMonitor(page, {
+    excludePatterns: [/analytics\.google\.com/, 'sentry.io'],
+  });
+  try {
+    await page.goto('/dashboard');
+    await expect(page.getByRole('heading', { name: 'Dashboard' })).toBeVisible();
+  } finally {
+    net.assertNoErrors(); // throws with method/status/url list if any 4xx/5xx
+  }
+});
+```
+
+Opt out of known 4xx paths via `excludePatterns` (substring or `RegExp`).
+Call `assertNoErrors()` (or `stop()`) so the listener is removed.