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, 2 insertions, 0 deletions

diff --git a/src/fen.c b/src/fen.c
index 2baa05d..37ce8da 100755
--- a/src/fen.c
+++ b/src/fen.c
@@ -301,6 +301,7 @@ Return value:
                 //
                 pos->cPawns[uColor][uPieceCounters[uPieceIndex]] = cSquare;
                 pos->bbPawns[uColor] |= COOR_TO_BB(cSquare);
+                pos->bbOccupied |= COOR_TO_BB(cSquare);
                 pos->rgSquare[cSquare].uIndex = uPieceCounters[uPieceIndex];
                 pos->rgSquare[cSquare].pPiece = p;
                 pos->uPawnMaterial[uColor] += VALUE_PAWN;
@@ -332,6 +333,7 @@ Return value:
                     uNumNonPawns[uColor]++;
                     pos->bbPieces[uColor][PIECE_TYPE(p)] |= COOR_TO_BB(cSquare);
                 }
+                pos->bbOccupied |= COOR_TO_BB(cSquare);
                 pos->rgSquare[cSquare].pPiece = p;
                 pos->rgSquare[cSquare].uIndex = uIndex;
                 pos->uNonPawnMaterial[uColor] += PIECE_VALUE(p);