tests: BUG-4 RED locked (hide-HP clobbers activeDisplay), add write recorder

src/tests/HideHpToggle.test.js: renders App, selects campaign, toggles
hide-player-HP switch, asserts setDoc data includes activeCampaignId +
activeEncounterId. RED: data only {hidePlayerHp:true}, both clobbered.

Root cause proven with evidence (recorder):
  setDoc(activeDisplay/status, {hidePlayerHp:true}, {merge:true})
  data written = {hidePlayerHp:true} ONLY
  activeCampaignId = undefined
  activeEncounterId = undefined

setDoc = replace per contract. {merge:true} arg ignored. Toggle wipes
encounter pointer → DisplayView reads null → 'Game Session Paused'.
Fix: use updateDoc (patch), not setDoc.

src/storage/firebase.js: adapter recorder now captures setDoc + updateDoc
(data + opts). Was subscribe-only. Enables write-path assertions.

src/tests/HideHpToggle.test.js

@@ -0,0 +1,63 @@
+// BUG-4 repro: toggling hidePlayerHp must not clobber activeDisplay doc.
+// setDoc = replace (contract). {merge:true} arg ignored.
+// Toggling hide-HP writes {hidePlayerHp:X} alone → activeCampaignId + activeEncounterId → null.
+// Display reads null → "Game Session Paused". Recover requires re-activating encounter.
+// Fix: use updateDoc (patch), not setDoc.
+
+import React from 'react';
+import { render, waitFor, screen, fireEvent } from '@testing-library/react';
+import '@testing-library/jest-dom';
+import App from '../App';
+import { MOCK_DB } from '../__mocks__/firebase/_mock-db';
+import { getAdapterCalls, resetAdapterCalls } from '../storage/firebase';
+import { selectCampaignByName } from './testHelpers';
+
+function seedAdminWithActiveEncounter() {
+  const campaignPath = 'artifacts/ttrpg-initiative-tracker-default/public/data/campaigns/c1';
+  const encounterPath = 'artifacts/ttrpg-initiative-tracker-default/public/data/campaigns/c1/encounters/e1';
+  const activeDisplayPath = 'artifacts/ttrpg-initiative-tracker-default/public/data/activeDisplay/status';
+
+  MOCK_DB.set(campaignPath, { name: 'Camp', playerDisplayBackgroundUrl: '' });
+  MOCK_DB.set(encounterPath, { name: 'Enc', participants: [], isStarted: true });
+  // active encounter set, HP NOT hidden
+  MOCK_DB.set(activeDisplayPath, {
+    activeCampaignId: 'c1',
+    activeEncounterId: 'e1',
+    hidePlayerHp: false,
+  });
+}
+
+describe('BUG-4: hide-player-HP toggle preserves activeDisplay', () => {
+  beforeEach(() => {
+    window.history.replaceState({}, '', '/');
+    global.alert = jest.fn();
+    resetAdapterCalls();
+  });
+
+  test('toggling hidePlayerHp does NOT clear activeCampaignId/activeEncounterId', async () => {
+    seedAdminWithActiveEncounter();
+    render(<App />);
+
+    // wait for admin to mount + load active display
+    await waitFor(() => screen.getByText('Camp'), { timeout: 3000 });
+    await selectCampaignByName('Camp');
+
+    // find the hide-player-HP toggle (role switch)
+    const toggle = await screen.findByRole('switch', { name: /hide/i }, { timeout: 3000 });
+
+    // toggle ON
+    fireEvent.click(toggle);
+
+    await waitFor(() => {
+      const writes = getAdapterCalls().filter(
+        c => c.fn === 'setDoc' && c.path.includes('activeDisplay/status')
+      );
+      expect(writes.length).toBeGreaterThan(0);
+      const last = writes[writes.length - 1];
+      // data written must include activeCampaignId AND activeEncounterId
+      // BUG: writes only {hidePlayerHp:true}, clobbering them.
+      expect(last.data.activeCampaignId).toBe('c1');
+      expect(last.data.activeEncounterId).toBe('e1');
+    }, { timeout: 3000 });
+  });
+});