Function decode_coding_tree_cabac 

pub fn decode_coding_tree_cabac(
    state: &mut HevcSliceCabacState<'_>,
    x: usize,
    y: usize,
    log2_cu_size: u8,
    depth: u8,
    max_depth: u8,
    sps: &HevcSps,
    pps: &HevcPps,
    slice_type: HevcSliceType,
    pic_width: usize,
    pic_height: usize,
    recon_luma: &mut Vec<i16>,
    results: &mut Vec<DecodedCu>,
    dpb: &HevcDpb,
    mv_field: &mut Vec<HevcMvField>,
)

Recursively decode a coding tree using CABAC, producing decoded CU leaves.

This replaces the stub decode_coding_tree in hevc_decoder.rs with actual CABAC-driven split decisions and CU parsing.