[idd] ipc: connect LGInput to LGIdd

Move the reusable named-pipe endpoint and shared support code into
the LGCommon static library.

Run a dedicated server in LGIdd and a reconnecting client in
LGInput, with device-lifecycle handling and report framing.

Refactor the helper pipe to use the same endpoint implementation.

idd/LGCommon/InputPipeProtocol.h

@@ -0,0 +1,51 @@
+/**
+ * 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 <stddef.h>
+#include <stdint.h>
+
+static constexpr wchar_t LG_INPUT_PIPE_NAME[] =
+  L"\\\\.\\pipe\\LookingGlassIDDInput";
+
+static constexpr uint32_t LG_INPUT_PIPE_MAGIC = 0x5049474c;
+static constexpr uint16_t LG_INPUT_PIPE_VERSION = 1;
+static constexpr size_t LG_INPUT_PIPE_MAX_REPORT_SIZE = 64;
+
+enum LGInputPipeMessageType : uint16_t
+{
+  LG_INPUT_PIPE_MESSAGE_REPORT = 1,
+};
+
+#pragma pack(push, 1)
+struct LGInputPipeMessage
+{
+  uint32_t magic;
+  uint16_t version;
+  uint16_t type;
+  uint32_t payloadSize;
+  uint64_t sequence;
+  uint8_t payload[LG_INPUT_PIPE_MAX_REPORT_SIZE];
+};
+#pragma pack(pop)
+
+static_assert(sizeof(LGInputPipeMessage) == 84,
+  "LGInputPipeMessage wire layout changed");