feat: U8x64 byte-level ops for palette codec, nibble, byte scan (Pumpkin/SD) Added to all three tiers (AVX-512 / AVX2 / scalar): cmpeq_mask(other) → u64 — byte-wise equality, returns bitmask shr_epi16(imm) → Self — shift right 16-bit lanes (nibble extract) saturating_sub(other) — max(a-b, 0) per byte (delta subtraction) unpack_lo_epi8(other) — interleave low bytes (nibble interleave) unpack_hi_epi8(other) — interleave high bytes These operations are used by: palette_codec.rs — Minecraft-style variable-width bit packing nibble.rs — 4-bit light level packing (Pumpkin) byte_scan.rs — NBT format byte scanning (future) stable_diffusion/ — VAE latent palette encoding via GGUF All three are currently using raw _mm256_/_mm512_ intrinsics. Next step: rewire them to use crate::simd::U8x64 instead. https://claude.ai/code/session_01ChLvBfpJS8dQhHxRD4pYNp#76