Add GETATTACKS_BITBOARD toggle, wiring _GetAttacksBB into real search

Board-representation migration section 6: a GNUmakefile build flag
(-DGETATTACKS_BITBOARD) makes chess.h's GetAttacks macro resolve to
_GetAttacksBB instead of the real asm implementation (or SlowGetAttacks
under CROUTINES) -- a three-way choice at the same spot the existing
CROUTINES switch already lived. _GetAttacksBB is now reachable from
every real call site (generate.c's check-detection call, see.c's
SEE(), searchsup.c), not just the test/bench harness.

Found and fixed while verifying this: testsee.c's TestGetAttacks and
its benchmark call the identifier GetAttacks meaning "the real
asm/CROUTINES baseline" -- once the macro could resolve to
_GetAttacksBB, those calls would silently compare the new
implementation against itself, turning both the correctness sweep and
the benchmark into false-positive no-ops. Fixed with a local #undef
GetAttacks right after #include "chess.h" in testsee.c, so the harness
always validates against the true baseline regardless of which
implementation is live in production.

Verified: gmake TEST=1 GETATTACKS_BITBOARD=1 passes (self-test suite,
corrected benchmark still reporting real asm vs. _GetAttacksBB
correctly, and a real Search() call exercising _GetAttacksBB live).
precommit_check.sh GETATTACKS_BITBOARD=1 clean for both the TEST=1
self-test and DEBUG=1 smoke test. Default (no flag) build confirmed
unaffected -- GetAttacks still resolves to the real asm function.

See board_representation/MIGRATION.md section 6 for the full writeup.
Sections 4/5/7's remaining items (curated-suite sd10 comparison,
match_play.py gate) are now unblocked but not yet run.

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

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