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

main.cpp

@@ -2896,8 +2896,9 @@ void set_attribute_value(const char *arg) {
 			exit(EXIT_JSON);
 		}
 
-		size_t i = 0;
-		for (const auto &e : o->entries()) {
+		for (size_t i = 0; i < o->entries().size(); i++) {
+			const auto &e = o->entries()[i];
+
 			if (e.key->type != JSON_STRING) {
 				fprintf(stderr, "%s: --set-attribute %s: key %zu not a string\n", *av, arg, i);
 				exit(EXIT_JSON);
@@ -2905,7 +2906,6 @@ void set_attribute_value(const char *arg) {
 
 			serial_val val = stringify_value(e.value.get(), "json", 1, o.get());
 			set_attributes.emplace(e.key->string(), val);
-			i++;
 		}
 
 		return;