Retire ATTACK_BITV/bvAttacks and the c|8 shadow-index mechanism entirely

Now that king (the last piece writing it) has converted to a bitboard
accumulator, nothing writes rgSquare[c|8].bvAttacks any more -- deletes
the whole mechanism rather than leaving a known-dead struct around:

- chess.h: ATTACK_BITV union gone. SQUARE collapses from a
  union-with-ATTACK_BITV to a plain {pPiece, uIndex} struct (the #pragma
  pack(1) that only existed for ATTACK_BITV's bitfield layout goes too).
  UNSAFE_FOR_ROOK/UNSAFE_FOR_QUEEN and the whole-word MINOR_XRAY_BIT/
  ROOK_XRAY_BIT/QUEEN_XRAY_BIT constants deleted -- confirmed unused
  (only ever referenced in stale comments, not code) now that every
  consumer reads bbXAttacks bitboards directly. PAWN_BIT/MINOR_BIT/
  ROOK_BIT/QUEEN_BIT/KING_BIT stay: they're a separate, still-live
  local bit-packing scheme _EvalKing/_WhoControlsSquareFast use to
  build a per-square attack-pattern index into KING_COUNTER_BY_
  ATTACK_PATTERN/g_SwapTable, unrelated to the retired storage struct.

- eval.c: _ClearAttackTables drops its entire macro-unrolled,
  128-square clearing loop (CLEAR_A_SQ/CLEAR_A_RANK/CLEAR_SHORT_RANK,
  all deleted) -- it only ever existed to zero the old per-square
  struct; clearing the 7 bbXAttacks accumulators is the whole function
  now. The transitional _IsSquareAttackedByX/_IsSquareXrayedByX helpers
  (minor/rook/queen/king, 8 functions total) are deleted outright, not
  just simplified -- their only remaining purpose was bridging to the
  now-gone struct, and their DEBUG cross-checks were explicitly
  migration-only scaffolding, not a permanent invariant. Call sites
  (_WhoControlsSquareFast, _EvalKing's bvAttack/bvXray/bvDefend) read
  the bbXAttacks bitboards directly instead. _WhoControlsSquareFast
  simplifies to a flat OR of 8 bitboard membership tests per color,
  down from raw struct reads plus 7 helper calls each.

Found and fixed one real, pre-existing bug while doing this (flagged
and confirmed with the user before touching it, kept as its own
documented change rather than silently folded into the mechanical
rename): _EvaluateCandidatePasser's helper-pawn-safety gate read
rgSquare[c1+8].bvAttacks[...].uWholeThing, but this function runs from
_EvalPawns -- the first piece type Eval() evaluates each call, before
any non-pawn piece (or, since commit 57502d6 retired pawns' own
bvAttacks write, even pawns) has written anything there. That word has
therefore been unconditionally zero, and the gate unconditionally true
(a silent no-op), since 57502d6 landed -- not something today's cleanup
introduced. Left exactly as dead/unconditional (deleted the
now-meaningless condition, kept the body it always ran anyway) rather
than fixed, since a real fix changes eval scoring and deserves its own
before/after check, documented inline for a future session.

Verification: precommit_check.sh (self-test + DEBUG smoke test) passes.
tests/ecm_ringers.ep_ at sd10 vs. the immediately preceding commit:
solve parity holds exactly (10/11 both), and final (depth-10) node
counts are byte-identical for all 11 positions -- the bar for a change
meant to be purely mechanical, unlike king's own conversion. One
harmless artifact noted: ECM.750 has a different depth-6 *intermediate*
best move (a shallow tie-break flip) that already resolves to the
identical PV and node count by depth 7 and holds through depth 10 --
not chased further since the actual (depth-10) result matches exactly
and search is deterministic at --cpus 1.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XxmVi2sTMwpPp4i6WYFjan

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