Function update_actions_commitment 

pub fn update_actions_commitment(
    prev: &[u8; 32],
    actions_root: &[u8; 32],
    height: u32,
) -> [u8; 32]

Update the running actions commitment chain.

chain_i = BLAKE2b-256("ZYNC_actions_v1" || chain_{i-1} || actions_root_i || height_i)

This mirrors the existing update_state_commitment pattern in header_chain.rs.