see.c: replace _WhoAttacksSquareBB's ray-walk with magic-bitboard lookups

_RookAttacksBB/_BishopAttacksBB (generate.c) are already O(1)
FORCEINLINE magic-table lookups, proven and already used everywhere
else in this codebase (eval.c, searchsup.c, generate.c's own move
generation) -- this function alone predated that migration and was
left on a manual per-direction ray-walk (4 rook + 4 bishop directions,
nearest-blocker isolation via FastFirstBit/FastLastBit). King handling
is also branchless now: king adjacency is symmetric, so
g_KingAttacksBB[cSquare] doubles as "every square a king standing
there would attack cSquare from," same O(1)-table-lookup shape the
knight line already uses -- no DISTANCE()==1 branch needed.

Old ray-walk kept as _WhoAttacksSquareRayBB, testsee.c's
TestWhoAttacksSquareMagic correctness-diff reference (20000 random
positions x every square x both colors, clean) -- same convention as
_AddXRaysMailbox next to _AddXRaysBB from an earlier SEE investigation
this session.

Isolated benchmark: magic is 0.95x-0.99x ray-walk in a standalone
3-position microbenchmark (small, since the ray-walk already has its
own early-out for "no slider anywhere near this square," per its own
header comment). Measured end-to-end (sd10 + sn5M, all three curated
suites, before vs. after): node counts and solve counts bit-for-bit
identical (pure algorithmic swap, zero behavior change). Wall-clock
~0.8% faster overall, all 6 measurements faster, none slower.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

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