Retire pawns' bvAttacks writes in favor of pos->bbPawnAttacks[2]

First piece type converted per board_representation/EVAL.md section 2:
pawns no longer write into rgSquare[c|8].bvAttacks at all. Every
consumer of "does a pawn attack this square" now reads
pos->bbPawnAttacks[2] directly -- a plain bitboard, computed fresh
each Eval() call from pos->bbPawns[] via the same shift-and-mask
technique generate.c's _GenerateAllPawnMovesBB already uses (zero
per-pawn mailbox iteration, vs. up to 16 delta+IS_ON_BOARD checks
before). Knight/bishop/rook/queen/king are unchanged -- still
populate/read their own bvAttacks bits (uMinor/uRook/uQueen/uKing)
the old way until their own conversions land.

Consumer changes, all in eval.c:
- UNSAFE_FOR_MINOR retired as a macro (it only ever tested the pawn
  bit) -- its 3 call sites (knight, bishop x2) now test
  bbPawnAttacks directly.
- UNSAFE_FOR_ROOK/_QUEEN masks narrowed to drop the now-dead pawn
  bit; call sites OR in an explicit bbPawnAttacks test alongside the
  narrowed bvAttacks read.
- Two direct .small.uPawn reads (bishop's transient-pawn mobility
  credit, bishop's defended-pawn bonus) switched to bbPawnAttacks
  tests.
- _EvalKing's bvAttack/bvDefend (the real king-danger computation)
  OR the bitboard bit back in at both read points, careful to
  preserve the original ordering where bvDefend must reflect the
  king's own just-set defend bit.
- _WhoControlsSquareFast (used by passer-race/trapped-piece/danger
  code) ORs the bitboard bit back into its g_SwapTable index at the
  same bit position PAWN_BIT always occupied.
- Two bugs caught by manually auditing every remaining |8 site after
  the fact (not by any test failing): _EvalPawns' own pawn-duo and
  backward-pawn detection read bvAttacks.uWholeThing at a point in
  Eval()'s sequence where only pawns could have written it -- once
  pawns stopped writing there, both checks went permanently dead
  silently. Fixed to read bbPawnAttacks directly. No self-test caught
  this; it's exactly the gap EVAL.md section 5's planned exact-score
  harness is meant to close.

EVAL_TIME instrumentation: split _EvalPawns' cycle counter into
pawn-hash hit/miss buckets (chess.h/eval.c/root.c), answering whether
the hash is still worth it now that attack-bit population is nearly
free. Measured on one sd12 benchmark position: hits average 90.4
cycles, misses average 1741.3 cycles (~19x), 97.15% hit rate -- the
hash stays a clear win; the miss cost was never mostly attack-bit
population (that's a separate ~1% bucket now, down from ~3.6%), it's
the isolated/doubled/duo/backward-pawn scoring loops, which still do
real per-pawn work on a miss.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01P6g6iF6mD1Hau6nCZCzwYj

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