Maintain pos->bbOccupied incrementally, dedupe its two from-scratch builders

chess.h: add POSITION::bbOccupied (full-board occupancy, both colors,
every piece including kings), maintained incrementally alongside
bbPieces/bbPawns rather than rebuilt on demand -- resolves the open
question in EVAL.md section 0 about whether this is worth doing given
both generate.c and the planned eval.c mobility rewrite need it.

move.c: SlidePiece/SlidePawn/LiftPiece/PlacePiece and their
WithoutSigs variants now maintain bbOccupied at the same choke points
that already maintain bbPieces/bbPawns -- unconditionally, since
occupancy doesn't care about piece type or color. Kings only ever
move through SlidePiece/SlidePieceWithoutSigs (never Lift/Place), so
no separate king-specific update site was needed.

fen.c: populate bbOccupied when parsing a FEN.

board.c: VerifyPositionConsistency cross-checks pos->bbOccupied
against a from-scratch rebuild, same pattern already used for
bbPieces/bbPawns.

generate.c/movesup.c/see.c: replace call sites that rebuilt full
occupancy via _BuildFullOccupiedBB/_BuildOccupiedBB with direct reads
of pos->bbOccupied; delete see.c's _BuildOccupiedBB, which was a
byte-for-byte duplicate of generate.c's _BuildFullOccupiedBB (kept
only as the from-scratch ground truth for the new consistency check
and testgenerate.c's benchmark harness).

testsup.c: GenerateRandomLegalPosition builds POSITIONs by poking
rgSquare/bbPieces/bbPawns directly, bypassing both move.c and fen.c --
a third construction path the above missed. It never set bbOccupied,
so the new VerifyPositionConsistency check failed on every generated
position, and since generation retries until a position verifies, the
self-test suite spun forever (100% CPU, no progress) instead of
crashing outright. Fixed by setting bbOccupied at all four placement
sites (both kings, pawn, non-pawn piece). Full self-test suite and
precommit_check.sh verified clean afterward.

board_representation/EVAL.md: record the bbOccupied decision and
rationale, resolving section 0's open question.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01P6g6iF6mD1Hau6nCZCzwYj

1 files changed, 8 insertions, 35 deletions

diff --git a/src/see.c b/src/see.c
index 4ba62ad..dc308d5 100755
--- a/src/see.c
+++ b/src/see.c
@@ -167,39 +167,12 @@ Return value:
 // out-of-line FirstBit/LastBit -- worth avoiding call overhead in a
 // per-move-generated, per-node hot path like this one.
 //
-static BITBOARD
-_BuildOccupiedBB(IN POSITION *pos)
-/**
-
-Routine description:
-
-    Full-board occupancy (both colors, every piece including pawns
-    and kings), built from the incrementally-maintained bbPieces[2][8]
-    and bbPawns[2] fields plus the king mailbox array
-    (cNonPawns[.][0], a single square per side -- a bitboard for that
-    adds nothing). All O(1) ORs now that bbPawns exists; this used to
-    loop cPawns[2][8] (up to 16 iterations) to build the pawn portion,
-    which ran on every single call regardless of how few pawns were
-    actually relevant.
-
-Parameters:
-
-    POSITION *pos
-
-Return value:
-
-    BITBOARD
-
-**/
-{
-    return (pos->bbPieces[WHITE][KNIGHT] | pos->bbPieces[WHITE][BISHOP] |
-            pos->bbPieces[WHITE][ROOK]   | pos->bbPieces[WHITE][QUEEN]  |
-            pos->bbPieces[BLACK][KNIGHT] | pos->bbPieces[BLACK][BISHOP] |
-            pos->bbPieces[BLACK][ROOK]   | pos->bbPieces[BLACK][QUEEN]  |
-            pos->bbPawns[WHITE]          | pos->bbPawns[BLACK]          |
-            COOR_TO_BB(pos->cNonPawns[WHITE][0]) |
-            COOR_TO_BB(pos->cNonPawns[BLACK][0]));
-}
+// (This file used to have its own static _BuildOccupiedBB here,
+// byte-for-byte identical to generate.c's _BuildFullOccupiedBB --
+// removed 2026-09-05 now that pos->bbOccupied is incrementally
+// maintained directly on POSITION; see board_representation/
+// EVAL.md section 0. Callers below just read pos->bbOccupied.)
+//
 
 // Non-static (unlike its historical file-local status) so
 // generate.c's Part B (_GenerateEscapes) bitboard work can call it
@@ -249,7 +222,7 @@ Parameters:
     POSITION *pos,
     COOR cSquare : target square
     ULONG uSide : side whose attackers on cSquare we want
-    BITBOARD bbOccupied : full-board occupancy (see _BuildOccupiedBB)
+    BITBOARD bbOccupied : full-board occupancy (see pos->bbOccupied)
 
 Return value:
 
@@ -412,7 +385,7 @@ Return value:
     //
     // Knights/bishops/rooks/queens/king, via the bitboard primitive.
     //
-    bbOccupied = _BuildOccupiedBB(pos);
+    bbOccupied = pos->bbOccupied;
     bbAttackers = _WhoAttacksSquareBB(pos, cSquare, uSide, bbOccupied);
     while (bbAttackers)
     {