Potentially unaligned 64-bit write constructing transport AEAD nonce

Summary

Transport nonce built via *(uint64_t*)(nonce+4)=htole64(counter)(:996-997,:1024-1025) on stack uint8_t nonce[12] with no alignment guarantee. Strict-aliasing UB. Transparent on x86-64 (primary target). Portability concern only — bus error on ARM/SPARC/MIPS strict-alignment ports. No security impact. Fix: use memcpy.