Fix _EvaluateCandidatePasser's helper-pawn safety gate (real no-op since 57502d6)

Flagged during the ATTACK_BITV/bvAttacks cleanup (5405191) but left
unfixed there deliberately, per direct instruction, since a real fix
changes eval scoring and deserves its own before/after check rather
than being buried in a mechanical rename commit.

The gate ("is this square safe to advance a helper pawn into, i.e.
not enemy-attacked or already friend-defended") used to read the old
combined bvAttacks word for both colors at the target square. This
function runs from _EvalPawns, the first piece type Eval() evaluates
each call -- no non-pawn piece (and, since commit 57502d6, not even
pawns themselves) has written any attack data yet at this point. That
word has therefore been unconditionally zero, and the gate
unconditionally true (silently disabled), since 57502d6 landed.

Fixed using pos->bbPawnAttacks -- the one piece-type bitboard that
actually is valid this early in Eval()'s sequence (populated at the
top of _EvalPawns, before this function runs). Narrower than whatever
the original gate covered (pawns only, not every piece type), but a
real, correct check instead of a fake one, and pawns are the dominant
real-world case for contesting a helper-pawn's advance square anyway.
Applied to both the right- and left-side helper searches (the initial
target-square check and the backward walk-and-search loop each need
their own copy, since the loop's own square changes every iteration).

Verified via precommit_check.sh, then all three curated suites at
sd10 (not just ringers, since this is a real eval-scoring change, not
a mechanical one): net +2/191 (113->115), concentrated in
ecm_hard_quick (18->22) with a small ecm_confident_quick give-back
(85->83) and ringers unchanged (10/11) -- the same "gains lopsided
toward hard_quick" shape this session's other real eval changes have
shown, consistent with a genuine (if modest) positional improvement
rather than suite-specific noise.

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

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