PieceMobility

Shared-core primitive computing per-piece-type pseudo-legal move counts for an explicit perspective color.

Moves are computed on state.withActiveColor(color) with castling dice present (King and Rook), exactly matching the move-generation operands of RichFeatures (mobility_diff). The move counts follow dice-value order: Pawn (1), Knight (2), Bishop (3), Rook (4), Queen (5), and King (6).

The sum over all six piece types equals the total pseudo-legal move count for color, preserving the invariant: sum(own_moves_*) - sum(opp_moves_*) == mobility_diff across every board position.