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/trianglesrpc.cpp

@@ -73,7 +73,7 @@ void RPCTypeCheck(const Array& params,
         if (!((v.type() == t) || (fAllowNull && (v.type() == null_type))))
         {
             string err = strprintf("Expected type %s, got %s",
-                                   Value_type_name[t], Value_type_name[v.type()]);
+                                   ValueTypeName(t), ValueTypeName(v.type()));
             throw JSONRPCError(RPC_TYPE_ERROR, err);
         }
         i++;
@@ -93,7 +93,7 @@ void RPCTypeCheck(const Object& o,
         if (!((v.type() == t.second) || (fAllowNull && (v.type() == null_type))))
         {
             string err = strprintf("Expected type %s for %s, got %s",
-                                   Value_type_name[t.second], t.first.c_str(), Value_type_name[v.type()]);
+                                   ValueTypeName(t.second), t.first.c_str(), ValueTypeName(v.type()));
             throw JSONRPCError(RPC_TYPE_ERROR, err);
         }
     }