[KnightStackSys]

Summary

• Implement pawn-structure-conditioned move history for quiet moves of all piece types, not only pawn moves, keyed by the pawn layouts of Red, Blue, Yellow and Green.
• Maintain Position::pawnKey() incrementally through pawn placements, captures, promotions, elimination and make/undo; side-to-move, castling and en passant metadata do not change the pawn signature.
• Index a bounded 256 × 32 × 160 (≈2.5 MiB) signed 16-bit history table by pawn hash bucket, full color-coded moving piece and destination square. Teammates have separate history entries.
• Reward quiet beta-cutoff moves, penalize failed quiet moves using existing history-bonus saturation. Integrate scores into PV move ordering and lazy non-PV move picking, with preexisting LMP/HMP handling.
• New SearchPawnHistoryWeight UCI option: 0–200%, default 50%; 0 disables both scoring and learning for A/B testing.
• Test pawn-key integrity after non-pawn moves, pawn pushes/captures/promotions, null moves and undo; verify tuning, score isolation/saturation and history clearing.
• Documentation in docs/pawn-history.md.

Notes

Does not include experimental knight/evaluation branches. No changes to main. CI pending; no Elo or NPS gain claimed. Compare against main with matched 4PC Teams SPRT and fixed-node speed testing.