Replace json_spirit with nlohmann/json via compatibility shim

Remove all json_spirit source files and add nlohmann/json (v3.11.3)
with a json_compat.h shim that preserves the json_spirit namespace
API. Updates all RPC and test files to use the new JSON backend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/test/script_tests.cpp

@@ -7,9 +7,7 @@
 #include <boost/algorithm/string/split.hpp>
 #include <boost/preprocessor/stringize.hpp>
 #include <boost/test/unit_test.hpp>
-#include "json/json_spirit_reader_template.h"
-#include "json/json_spirit_writer_template.h"
-#include "json/json_spirit_utils.h"
+#include "json/json_compat.h"
 
 #include "main.h"
 #include "wallet.h"