Function encode_patchable_mov_imm 

pub fn encode_patchable_mov_imm(
    rd: u32,
    is_64bit: bool,
    value: u64,
) -> SmallVec<[u8; 16]>

Encode a fixed-width patchable mov-immediate sequence for AArch64.

Always emits 2 instructions (8 bytes) for W registers and 4 (16 bytes) for X registers so rewrites never change layout.