Refactor encoder_iter_iternext into focused helper functions

Extract iter_handle_root, iter_handle_list_frame, iter_handle_dict_frame,
iter_open_child and iter_encode_unknown from the monolithic iternext body.
encoder_iter_iternext is now 25 lines; each helper has a single responsibility.
No behaviour change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: eendebakpt