Break the 666-line choose_swap_or_discard into 8 focused sub-functions,
extract named constants for ~15 magic numbers, add column/pair utility
functions (iter_columns, project_score, count_hidden, hidden_positions),
and extract _log_cpu_action helper to reduce logging boilerplate in
process_cpu_turn. No behavior changes - validated with simulate.py 500.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>