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.hpp

@@ -17,7 +17,7 @@ struct parse_json_args {
 	struct serialization_state *sst;
 
 	parse_json_args(json_pull_ptr jp1, int layer1, std::string *layername1, struct serialization_state *sst1)
-	    : jp(jp1), layer(layer1), layername(layername1), sst(sst1) {
+	    : jp(std::move(jp1)), layer(layer1), layername(layername1), sst(sst1) {
 	}
 };