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

src/lan_pairing.h

@@ -0,0 +1,26 @@
+#pragma once
+#include "platform.h"
+#include "../vendor/json.hpp"
+
+namespace ks::lan
+{
+using Json = nlohmann::json;
+inline constexpr unsigned Port = 45883;
+inline constexpr uint64_t PairLifetime = 120000, PeerLifetime = 12000;
+std::string randomHex(size_t bytes);
+std::string normalizeCode(const std::string &);
+std::string mac(const std::string &key, const std::string &text);
+bool equal(const std::string &, const std::string &);
+bool identifier(const std::string &);
+Json envelope(const std::string &type, const std::string &id);
+struct Pairing
+{
+    std::string local, peer, code, nonce, remoteNonce, derived;
+    uint64_t started{};
+    bool acknowledged{}, confirmed{};
+    Pairing(std::string local, std::string peer, const std::string &code, uint64_t now);
+    bool expired(uint64_t now) const;
+    Json offer() const;
+    std::vector<Json> receive(const Json &, uint64_t now);
+};
+}