Add bitboard-backed GetAttacks (section 2/3), verified faster than asm

Board-representation migration, sections 2-3 (GetAttacks half):

- board.c: VerifyPositionConsistency's bbPieces consistency check
  (migration section 2), verified clean via gmake TEST=1 with the
  assert live.
- POSITION.bbPawns[2]: new incrementally-maintained per-color pawn
  location bitboard (chess.h), maintained at the same 6 move.c sites
  as bbPieces, populated from scratch in fen.c. Distinct from the
  pawn-hash-keyed bbPawnLocations; this one needs no
  SEARCHER_THREAD_CONTEXT, so it's reachable from GetAttacks's actual
  call sites (which only ever have a POSITION*).
- data.c/chess.h/main.c: g_RookRayAll/g_BishopRayAll (all 4 per-square
  ray directions pre-ORed) and g_PawnAttackOriginBB[2][128] startup
  tables, plus FastFirstBit/FastLastBit (static inline bsf/bsr
  wrappers, chess.h) -- supporting tables/helpers for the primitive
  below.
- see.c: _WhoAttacksSquareBB (bitboard "who attacks square X" query)
  and _GetAttacksBB (SEE_LIST-populating PoC wrapping it), side by
  side with the existing SlowGetAttacks/asm GetAttacks -- not wired
  into the GetAttacks macro yet (section 6), pure addition.
- testsee.c: SeeListsAreEqual made order-independent (SEE() sorts the
  list right after GetAttacks returns, so order was never semantically
  significant); TestGetAttacks extended to run _GetAttacksBB as a
  third comparison across the existing 20,000-random-position sweep;
  added an interleaved asm/Slow/BB cycles-per-call benchmark across
  opening/middlegame/endgame positions.
- testsup.c: fixed GenerateRandomLegalPosition (used by the sweep
  above) to maintain bbPieces/bbPawns at its two hand-placement sites
  -- a latent gap since section 1 that made its own
  VerifyPositionConsistency legality gate almost always reject
  generated positions, causing large, variable retry-loop slowdowns.

Verified: 20,000-position x every-square x both-colors correctness
sweep passes (gmake TEST=1), precommit_check.sh clean (self-test +
DEBUG smoke test). Benchmark: _GetAttacksBB is ~0.53-0.55x asm
GetAttacks's cycles/call (opening/middlegame) and ~0.89x (endgame) --
faster, not just equivalent, primarily from replacing bbOccupied's
up-to-16-iteration pawn loop with two bbPawns ORs, plus a
g_PawnAttackOriginBB table lookup replacing per-call pawn-delta
arithmetic and per-direction/per-side-group early-outs in the slider
walk. See board_representation/MIGRATION.md section 3 for the full
writeup, including a reverted approach that measured slower and why,
and the CountKingSafetyDefects half's re-scoped (not yet implemented)
design.

Also confirmed (not caused by this work, not fixed here): a
pre-existing non-deterministic MP-race assertion in util.c:1093's PV
printing, reproduced independently on a clean HEAD checkout.

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

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