[idd] recovery: restore fallback display

Add a protocol-independent recovery channel outside LGMP so clients can
request a usable guest display even when transport versions differ.

Synchronize recovery state with the helper across driver restarts and
restore the configured topology without persisting temporary changes.

idd/LGIdd/transport/lgmp/CRecovery.h

@@ -0,0 +1,72 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#pragma once
+
+#include "CSRWLock.h"
+
+#include <stdint.h>
+
+struct KVMFRR;
+struct KVMFRRRequest;
+struct KVMFRRStatus;
+
+class CIVSHMEM;
+
+class CRecovery
+{
+public:
+  struct Request
+  {
+    uint64_t session = 0;
+    uint32_t serial  = 0;
+    bool     valid   = false;
+    bool     active  = false;
+  };
+
+private:
+  CSRWLock m_lock;
+  KVMFRR * m_data = nullptr;
+
+  uint64_t m_session       = 0;
+  uint64_t m_nextHeartbeat = 0;
+  uint64_t m_deadline      = 0;
+  uint32_t m_lastSerial    = 0;
+  uint32_t m_statusSerial  = 0;
+  uint32_t m_request       = 0;
+  bool     m_syncReady     = false;
+  bool     m_replay        = false;
+  bool     m_waiting       = false;
+
+  static bool ReadRequest(KVMFRRRequest& source, KVMFRRRequest& result);
+  static bool ReadStatus(KVMFRRStatus& source, KVMFRRStatus& result);
+  static bool SerialNewer(uint32_t serial, uint32_t reference);
+  bool OwnsSession();
+  uint32_t NextTicket();
+  void Publish(uint32_t serial, uint32_t request,
+    uint32_t state, uint32_t error);
+
+public:
+  bool Initialize(CIVSHMEM& ivshmem);
+  void Sync();
+  Request Process();
+  void SetStatus(uint64_t session, uint32_t serial, bool active,
+    uint32_t state, uint32_t error);
+};