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, 4 insertions, 4 deletions

diff --git a/src/movesup.c b/src/movesup.c
index 22bf0c1..82ba10a 100755
--- a/src/movesup.c
+++ b/src/movesup.c
@@ -220,7 +220,7 @@ Return value:
     iDelta = CHECK_DELTA_WITH_INDEX(iIndex);
     ASSERT(iDelta != 0);
 
-    bbOccupiedWithoutRemove = _BuildFullOccupiedBB(pos) & ~COOR_TO_BB(cRemove);
+    bbOccupiedWithoutRemove = pos->bbOccupied & ~COOR_TO_BB(cRemove);
     cBlocker = _NearestBlockerAlongRayBB(cLocation, iDelta,
                                          bbOccupiedWithoutRemove);
     return _ValidateExposedBlockerBB(pos, cBlocker, cLocation);
@@ -270,7 +270,7 @@ Return value:
     }
     iDelta = CHECK_DELTA_WITH_INDEX(iIndex);
 
-    bbOccupiedWithoutRemove = _BuildFullOccupiedBB(pos) & ~COOR_TO_BB(cRemove);
+    bbOccupiedWithoutRemove = pos->bbOccupied & ~COOR_TO_BB(cRemove);
     cBlocker = _NearestBlockerAlongRayBB(cLocation, iDelta,
                                          bbOccupiedWithoutRemove);
     return _ValidateExposedBlockerBB(pos, cBlocker, cLocation);
@@ -323,7 +323,7 @@ Return value:
     }
     iDelta = CHECK_DELTA_WITH_INDEX(iIndex);
 
-    bbOccupied = (_BuildFullOccupiedBB(pos) &
+    bbOccupied = (pos->bbOccupied &
                   ~COOR_TO_BB(cTest) & ~COOR_TO_BB(cIgnore)) |
                  COOR_TO_BB(cBlock);
     cBlocker = _NearestBlockerAlongRayBB(cKing, iDelta, bbOccupied);
@@ -641,7 +641,7 @@ Return value:
     ASSERT(IS_ON_BOARD(cTest));
     ASSERT(IS_VALID_COLOR(uSide));
 
-    bbOccupied = _BuildFullOccupiedBB(pos);
+    bbOccupied = pos->bbOccupied;
     if (0 != _WhoAttacksSquareBB(pos, cTest, uSide, bbOccupied))
     {
         return(TRUE);