- fix the no-op best-score update that ignored all but the first reply
- alternate sides correctly with negamax and add alpha-beta pruning
- score terminal wins and losses above material evaluations
- treat depth zero as one ply instead of underflowing
- guard relative_score against an empty board
- add hanging-piece, recapture, terminal, and depth regression tests
- make Position represent only valid board squares and use checked offsets
- make move endpoints fallible and remove unchecked coordinate arithmetic
- validate moves against generated legal moves, including mandatory capture
- track pending multi-captures and restrict continuation to the same piece
- derive pending state after undo/redo and preserve redo on invalid moves
- fix circular buffer wraparound, mixed iteration, and element drops
- adapt GTK and WASM to fallible endpoints and apply errors
- add rule, promotion, chain, history, and coordinate regression tests
woggioni