bug: bucket_id hash is non-deterministic on big-endian targets

[Troublor]

Summary

The bucket_id hash function produces platform-dependent results due to native-endian byte interpretation in the vendored AHash implementation. On big-endian targets, the same key bytes would produce different bucket IDs than on little-endian targets, violating the consensus determinism guarantee.

Severity

Low in practice (all current deployment targets are little-endian), but high in principle — a consensus-critical hash function must be deterministic across all platforms.

Affected Code

File: salt/src/state/ahash/convert.rs

macro_rules! convert {
    ($a:ty, $b:ty) => {
        impl Convert<$b> for $a {
            fn convert(self) -> $b {
                zerocopy::transmute!(self)  // ← native-endian bit reinterpretation
            }
        }
    };
}

zerocopy::transmute! is semantically equivalent to core::mem::transmute — it reinterprets the raw bits without any byte-order conversion. On a little-endian target, [0x01, 0x02, ..., 0x08] becomes 0x0807060504030201. On a big-endian target, the same bytes become 0x0102030405060708.

This feeds through ReadFromSlice::read_u64/u128/u32/u16 → Hasher::write → hash() → bucket_id(), meaning the same key produces different bucket IDs on LE vs BE.

Suggested Fix

Replace zerocopy::transmute! with explicit little-endian conversion:

// Before (native-endian, non-deterministic):
impl Convert<u64> for [u8; 8] {
    fn convert(self) -> u64 {
        zerocopy::transmute!(self)
    }
}

// After (always little-endian, deterministic):
impl Convert<u64> for [u8; 8] {
    fn convert(self) -> u64 {
        u64::from_le_bytes(self)
    }
}

Apply to all ReadFromSlice conversion paths: u16, u32, u64, u128.

This also allows dropping the zerocopy dependency entirely since it's only used for these transmute! calls in the hasher module.

Context

Discovered during megaeth-labs/mega-evm#225, where the salt crate's bucket_id hasher was inlined into mega-evm. The inlined version already uses from_le_bytes, making it deterministic across all platforms.