fix: end combat failing when character sync enabled

Character writeback in endEncounter wrote to a bare campaigns/{id} path,
which the firebase adapter passes straight to the SDK — the update hit a
nonexistent top-level doc, threw, and aborted the whole end-combat action
(round never reset, combat stayed running).

- endEncounter now uses ctx.campaignPath (full artifacts/... path from
  App.js), falling back to the bare path for server/test adapters
- writeback failures no longer block ending combat
- regression tests: campaignPath routing, writeback-failure resilience,
  and participants never cleared on end

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

shared/tests/turn.writeback.test.js

@@ -139,6 +139,55 @@ describe('character writeback on endEncounter', () => {
     expect(fighterOld.defaultCurrentHp).toBe(30);
   });
 
+  test('writeback uses ctx.campaignPath when provided (firebase prefixed paths)', async () => {
+    const storage = makeMockStorage();
+    const players = [{ id: 'char1', name: 'Fighter', defaultMaxHp: 30, defaultAc: 17, defaultCurrentHp: 30 }];
+    const prefixed = 'artifacts/app1/public/data/campaigns/camp1';
+    storage._docs.set(prefixed, { id: 'camp1', name: 'Camp', players, syncCharacters: true });
+    const enc = makeEncounter();
+    const ctx = {
+      storage,
+      encounterPath: 'campaigns/camp1/encounters/enc1',
+      campaignPath: prefixed,
+      logPath: 'logs/log1',
+      logCollection: 'logs',
+      displayPath: 'activeDisplay/status',
+    };
+    await endEncounter(enc, ctx);
+    expect(storage._writes.find(w => w.path === 'campaigns/camp1')).toBeUndefined();
+    const campWrite = storage._writes.find(w => w.path === prefixed);
+    expect(campWrite).toBeDefined();
+    expect(campWrite.patch.players.find(p => p.id === 'char1').defaultMaxHp).toBe(28);
+  });
+
+  test('writeback failure does not block ending combat', async () => {
+    const storage = makeMockStorage();
+    const players = [{ id: 'char1', name: 'Fighter', defaultMaxHp: 30, defaultAc: 17, defaultCurrentHp: 30 }];
+    storage._docs.set('campaigns/camp1', { id: 'camp1', name: 'Camp', players, syncCharacters: true });
+    storage.updateDoc = jest.fn(async (p, patch) => {
+      if (p === 'campaigns/camp1') throw new Error('No document to update');
+      storage._writes.push({ path: p, patch });
+    });
+    const enc = makeEncounter();
+    const ctx = {
+      storage,
+      encounterPath: 'campaigns/camp1/encounters/enc1',
+      encPath: 'campaigns/camp1/encounters/enc1',
+      logPath: 'logs',
+      logCollection: 'logs',
+      displayPath: 'activeDisplay/status',
+    };
+    const newEnc = await endEncounter(enc, ctx);
+    expect(newEnc.isStarted).toBe(false);
+    expect(newEnc.currentTurnParticipantId).toBe(null);
+    // participants are preserved — ending combat never clears the roster
+    expect(newEnc.participants).toHaveLength(3);
+    const encWrite = storage._writes.find(w => w.path === 'campaigns/camp1/encounters/enc1');
+    expect(encWrite).toBeDefined();
+    expect(encWrite.patch.isStarted).toBe(false);
+    expect(encWrite.patch.participants).toBeUndefined();
+  });
+
   test('writeback no-op if character missing from campaign', async () => {
     const storage = makeMockStorage();
     storage._docs.set('campaigns/camp1', { id: 'camp1', name: 'Camp', players: [], syncCharacters: true });