Convert jsonpull to C++ with shared_ptr and std::vector/std::string

Replace the manual malloc/realloc/free memory management in jsonpull
with std::shared_ptr ownership. Each json_object now owns its children
through std::vector<json_object_ptr>; raw back-pointers to parent and
parser remain valid by structural invariant and are cleared on
json_disconnect so detached subtrees can outlive their parser.
Strings become std::string, child arrays become std::vector, and the
old union becomes a struct so non-trivial members can coexist while
preserving the existing o->value.xxx access paths.

The old jsonpull.c is replaced by jsonpull.cpp, json_stringify now
returns std::string, and all callers across tippecanoe, tile-join,
tippecanoe-decode, tippecanoe-json-tool, tippecanoe-overzoom and the
unit tests are updated to use json_object_ptr / json_pull_ptr.

Co-authored-by: Cursor <cursoragent@cursor.com>

overzoom.cpp

@@ -238,7 +238,7 @@ int main(int argc, char **argv) {
 	std::string out;
 
 	{
-		json_object *json_filter = NULL;
+		json_object_ptr json_filter;
 		if (filter.size() > 0) {
 			json_filter = parse_filter(filter.c_str());
 		}