Tidy jsonpull call sites flagged in review

- geojson.cpp and attribute.cpp passed key_pool::pool() and
  set_attribute_accum() a c_str() from a std::string, forcing a needless
  reconstruction (and truncating at an embedded NUL). Both overloads take
  std::string, so pass it directly. The geojson.cpp one is the hottest
  loop in the program.
- Replace the hand-maintained counters beside range-for loops in
  attribute.cpp, main.cpp and tile-join.cpp with indexed loops, since the
  index is only wanted for error messages.
- parse_json_args took json_pull_ptr by value and then copied it, costing
  two refcount bumps per construction. Move it.
- Assert that the parser is still attached where geojson.cpp reads
  geometry->parser->line. Only json_read results reach it today, but
  json_read_tree and json_disconnect now clear every parser pointer, so a
  detached tree would null-deref there instead of tripping an assert.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KNxyHKasyWrWcvre2yK4r

geojson.cpp

@@ -188,7 +188,7 @@ int serialize_geojson_feature(struct serialization_state *sst, json_object *geom
 			if (e.key->type == JSON_STRING) {
 				serial_val sv = stringify_value(e.value.get(), sst->fname, sst->line, feature);
 
-				full_keys.emplace_back(key_pool.pool(e.key->string().c_str()));
+				full_keys.emplace_back(key_pool.pool(e.key->string()));
 				values.push_back(std::move(sv));
 			}
 		}
@@ -238,6 +238,10 @@ struct json_serialize_action : json_feature_action {
 	std::string layername;
 
 	int add_feature(json_object *geometry, bool geometrycollection, json_object *properties, json_object *id, json_object *tippecanoe, json_object *feature) {
+		// This only ever receives json_read results from geojson-loop, whose
+		// parser is still attached. json_read_tree / json_disconnect clear
+		// every parser pointer, so a detached tree would null-deref here.
+		assert(geometry->parser != nullptr);
 		sst->line = geometry->parser->line;
 		if (geometrycollection) {
 			int ret = 1;