Add opt-in KSwitcher LAN discovery and mutual code pairing preview

src/lan.h

@@ -0,0 +1,17 @@
+#pragma once
+#include "platform.h"
+#include <memory>
+
+namespace ks
+{
+// Local-only discovery and pairing. No input/clipboard receiver is exposed yet.
+class LanService
+{
+    struct Impl;
+    std::unique_ptr<Impl> impl;
+  public:
+    explicit LanService(const std::filesystem::path &);
+    ~LanService();
+    Bytes command(const Bytes &);
+};
+}