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, 19 insertions, 2 deletions

diff --git a/src/chess.h b/src/chess.h
index f39a956..41c8734 100755
--- a/src/chess.h
+++ b/src/chess.h
@@ -659,8 +659,8 @@ typedef struct _POSITION
 
     // Per-color pawn location bitboard -- same incremental-maintenance
     // idea as bbPieces above, but for pawns (which bbPieces
-    // deliberately excludes). Exists so _BuildOccupiedBB (see.c) can
-    // build full-board occupancy via two ORs instead of looping
+    // deliberately excludes). Exists so full-board occupancy
+    // (pos->bbOccupied below) can be built via two ORs instead of looping
     // cPawns[2][8] (up to 16 iterations) on every call -- see
     // board_representation/MIGRATION.md section 3. Also the single
     // source of truth for pawn locations used by pawn eval
@@ -671,6 +671,23 @@ typedef struct _POSITION
     // this field directly instead.
     BITBOARD bbPawns[2];
 
+    // Full-board occupancy (both colors, every piece including pawns
+    // and kings) -- same incremental-maintenance idea as bbPieces/
+    // bbPawns above, added 2026-09-05 per board_representation/
+    // EVAL.md section 0 (was previously only on MOVE_STACK, rebuilt
+    // on demand via _BuildFullOccupiedBB at many independent call
+    // sites in generate.c/movesup.c/see.c, and needed again by the
+    // planned eval.c mobility rewrite -- one incrementally-maintained
+    // copy here replaces all of that). Maintained by move.c's
+    // SlidePiece/SlidePawn/LiftPiece/PlacePiece (and their
+    // WithoutSigs variants) at the same sites that already maintain
+    // bbPieces/bbPawns, unconditionally (no per-color/per-type branch
+    // needed, unlike those). _BuildFullOccupiedBB(pos) (generate.c)
+    // remains as the from-scratch ground truth used to verify this
+    // field stays in sync (see VerifyPositionConsistency, board.c),
+    // not as something callers should call directly anymore.
+    BITBOARD bbOccupied;
+
     ULONG uWhiteSqBishopCount[2];          // num bishops on white squares
     SCORE iMaterialBalance[2];             // material balance