Fix no_std build for embedded targets

[loadingalias]

The Problem

Fixes #34 - no_std builds fail

Root Causes:

1. The fusion module compiles in no_std but depends on imports only available with std
2. avx512_vpclmulqdq.rs uses std::mem::transmute and println!
3. The spin crate is an unconditional dep, but requires atomics unavailable on some no_std targets (Cortex-M0, etc.)

The Solution

• Gate the fusion module on feature = "std"
• Use core::mem::transmute instead of std::mem::transmute
• Gate debug-only functions (print_hex, extract_u64s) with #[cfg(feature = "std")]
• Make spin a target-conditional dependency (only included on aarch64/x86/x86_64 where it's actually used)

Changes

The code already conditionally uses spin only on SIMD-capable architectures. This PR makes the dependency conditional to match, allowing builds on targets without atomics.

Planned version bump

• Which: PATCH
• Why: Non-breaking bug fix for no_std builds

Links

• Fixes Doesn't build on no_std with no features or alloc enabled only #34

Notes

Verified builds:

• x86_64-unknown-uefi (original issue target)
• thumbv6m-none-eabi (no atomics)
• Default features (all tests pass)
• CI is green