Add bbPieces incremental piece-location bitboards (migration plan section 1)

Board-representation migration, section 1: add POSITION.bbPieces[2][8]
(per-color, per-piece-type location bitboards, indexed like the
existing uNonPawnCount) as incrementally-maintained state, not a
per-Eval()-call rebuild -- the structural fix for why the earlier
attack-presence-bitboard work measured slower, not faster.

- chess.h: bbPieces[2][8] field; extern decls for data.c's
  g_RookRayToEdge/g_BishopRayToEdge/g_KnightAttacksBB ray tables and
  their Initialize* functions (needed by the planned bitboard-backed
  GetAttacks/CountKingSafetyDefects primitive, section 3).
- fen.c: populate bbPieces during piece placement; zeroing is free via
  the existing memset(p, 0, sizeof(POSITION)).
- move.c: maintain bbPieces at all 6 non-pawn piece-movement functions
  (SlidePiece/LiftPiece/PlacePiece and their WithoutSigs siblings used
  by UnmakeMove) -- covers every move type: normal moves, captures,
  both-side castling, promotion with/without capture, en passant, and
  every undo.
- board.c: extend VerifyPositionConsistency's existing non-pawn
  piece-list walk with a parallel bbPieces reconstruction-and-compare,
  rather than a separate bespoke check.
- data.c/main.c: pulled ray-to-edge/knight-attack tables from stash
  (needed by section 3, not section 1 itself, but zero-risk to land
  now).

Also, while verifying: COOR_TO_BB was a table lookup (BBSQUARE[idx])
measured ~5-7% slower than the pure-ALU shift already sitting unused in
SLOWCOOR_TO_BB (whose "SLOW" name reflects a stale assumption about
variable shifts never actually tested on this hardware). Switched
COOR_TO_BB to the shift; fixed testbitboard.c's existing but broken
(dead-code-eliminated, silently reporting "0 cycles/op") comparison
benchmark for both while at it.

Verified via gmake TEST=1 (including TestMakeUnmakeMove's explicit
en-passant/promotion-with-capture/both-castling coverage) and
debug_smoke_test.sh, both clean; release build clean and runs normally.
Nothing reads bbPieces yet -- pure addition, zero behavioral risk.

See board_representation/MIGRATION.md for the full plan.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Jntky4yGUTyQVaGCXms4F2

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