cmov

Conditional move CPU intrinsics which are guaranteed on major platforms (ARM32/ARM64, x86/x86_64, RISC-V) to execute in constant-time and not be rewritten as branches by the compiler. Provides wrappers for the CMOV family of instructions on x86/x86_64 and CSEL on AArch64, along with a portable "best-effort" pure Rust fallback implementation.