refactor(V2): IBKR WebSocket — fix stop/start cycle, guard rails, log disconnect

Code review fixes (commit 17700d2):
- _stopped reset on start() (was stuck True after stop→start)
- _require_started guard on subscribe_*/unsubscribe (clear WSError vs AttributeError)
- _reader_loop logs disconnect via logger.warning + sets _ws=None for `connected` signal
- Class docstring documents stale-snapshot behavior + deferred reconnect
- New tests: subscribe-before-start, stop→start cycle resumption

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/exchanges/ibkr/test_ws.py

@@ -84,3 +84,41 @@ async def test_subscribe_limit(fake_signer, monkeypatch):
     with pytest.raises(WSError, match="IBKR_WS_SUB_LIMIT"):
         await ws.subscribe_tick(3)
     await ws.stop()
+
+
+@pytest.mark.asyncio
+async def test_subscribe_before_start_raises(fake_signer):
+    ws = IBKRWebSocket(
+        signer=fake_signer,
+        ws_url="wss://x", base_url="https://x",
+        max_subs=10, idle_timeout_s=300,
+    )
+    with pytest.raises(WSError, match="IBKR_WS_NOT_STARTED"):
+        await ws.subscribe_tick(1)
+
+
+@pytest.mark.asyncio
+async def test_start_after_stop_resumes_reader(fake_signer, monkeypatch):
+    fake_ws_a = FakeWS()
+    fake_ws_b = FakeWS()
+    fakes = iter([fake_ws_a, fake_ws_b])
+
+    async def fake_connect(url, **kw):
+        return next(fakes)
+
+    monkeypatch.setattr("cerbero_mcp.exchanges.ibkr.ws.websockets_connect", fake_connect)
+
+    ws = IBKRWebSocket(
+        signer=fake_signer,
+        ws_url="wss://x", base_url="https://x",
+        max_subs=10, idle_timeout_s=300,
+    )
+    await ws.start()
+    await ws.stop()
+    # Restart with fresh fake_ws_b
+    await ws.start()
+    await ws.subscribe_tick(42)
+    await fake_ws_b.push({"topic": "smd+42", "31": "100"})
+    await asyncio.sleep(0.05)
+    assert ws.get_tick_snapshot(42) is not None
+    await ws.stop()