Add nlohmann::json library

src/vendor/nlohmann/json.cppm

@@ -0,0 +1,51 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0 (develop branch)
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+module;
+
+// GCC workaround for C++ modules support.
+// When using C++20 modules, some compilers (particularly GCC) may have issues
+// with template instantiations in the module preamble. If you encounter
+// "redefinition" errors when including nlohmann/json.hpp, try one of:
+// 1. Include nlohmann/json.hpp in your module preamble BEFORE other #includes
+// 2. Or use: import nlohmann.json;  instead of #include <nlohmann/json.hpp>
+// 3. Or upgrade to a newer GCC version with better modules support.
+// See: https://github.com/nlohmann/json/issues/5103
+
+#include <nlohmann/json.hpp>
+
+// NOTE: Module renamed from nlohmann.json to match directory structure
+export module vendor.nlohmann.json;
+
+export NLOHMANN_JSON_NAMESPACE_BEGIN
+    using NLOHMANN_JSON_NAMESPACE::adl_serializer;
+    using NLOHMANN_JSON_NAMESPACE::basic_json;
+    using NLOHMANN_JSON_NAMESPACE::json;
+    using NLOHMANN_JSON_NAMESPACE::json_pointer;
+    using NLOHMANN_JSON_NAMESPACE::ordered_json;
+    using NLOHMANN_JSON_NAMESPACE::ordered_map;
+    using NLOHMANN_JSON_NAMESPACE::to_string;
+
+    inline namespace literals
+    {
+        inline namespace json_literals
+        {
+            using NLOHMANN_JSON_NAMESPACE::literals::json_literals::operator""_json;
+            using NLOHMANN_JSON_NAMESPACE::literals::json_literals::operator""_json_pointer;
+        } // namespace json_literals
+    } // namespace literals
+
+    // Note: the following nlohmann::detail symbols must be exported due to
+    // an MSVC bug failing to compile without these symbols visible (ticket #3970)
+    namespace detail
+    {
+        using NLOHMANN_JSON_NAMESPACE::detail::json_sax_dom_callback_parser;
+        using NLOHMANN_JSON_NAMESPACE::detail::unknown_size;
+    } // namespace detail
+
+NLOHMANN_JSON_NAMESPACE_END