Expand description
PEXT-based sliding piece attacks (BMI2 instruction).
When BMI2 is available, the pext instruction replaces the magic
multiplication + shift with a single instruction, giving ~2× speedup
for sliding piece attacks.
Tables are built at init time using a software PEXT emulation, so
they work regardless of CPU support. The hot-path lookup uses hardware
PEXT when available via #[target_feature(enable = "bmi2")].
On non-x86_64 the entire module is dead code (the caller in attacks
is gated behind #[cfg(target_arch = "x86_64")]).