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

tile-join.cpp

@@ -973,8 +973,9 @@ void handle_strategies(const unsigned char *s, std::vector<strategy> *st) {
 		for (size_t i = 0; i < o->array().size(); i++) {
 			const json_object_ptr &h = o->array()[i];
 			if (h->type == JSON_HASH) {
-				size_t j = 0;
-				for (const auto &kv : h->entries()) {
+				for (size_t j = 0; j < h->entries().size(); j++) {
+					const auto &kv = h->entries()[j];
+
 					if (kv.key->type != JSON_STRING) {
 						fprintf(stderr, "Key %zu of %zu is not a string: %s\n", j, i, s);
 					} else if (kv.value->type != JSON_NUMBER) {
@@ -1005,7 +1006,6 @@ void handle_strategies(const unsigned char *s, std::vector<strategy> *st) {
 							(*st)[i].feature_count += kv.value->number();
 						}
 					}
-					j++;
 				}
 			} else {
 				fprintf(stderr, "Element %zu is not a hash: %s\n", i, s);