docs: add implementation plan for reducing exported functions (#77)

9 tasks covering: internalize reduction structs, unitdiskmapping internals,
polynomial/truth_table/graph_types modules, add is_valid_solution methods,
internalize validation free functions, config consolidation, prelude update.

Updated for PR #76 (problem_size trait additions).

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

Author: GiggleLiu