Function identity_top_up_from_shielded_extra_sighash_data 

pub fn identity_top_up_from_shielded_extra_sighash_data(
    identity_id: &[u8; 32],
    top_up_amount: u64,
    platform_version: &PlatformVersion,
) -> Result<Vec<u8>, ProtocolError>

Builds the transparent extra_data bound into an IdentityTopUpFromShieldedPool’s platform sighash, with the byte layout identity_id (32) || top_up_amount (u64 LE).

Like Unshield, the transition carries no platform signature, so the state-determining transparent fields (which identity is credited, and the gross amount leaving the pool) must be committed into the Orchard binding sighash; otherwise a relayer could take a valid spend bundle and re-point it at a different identity. The client builder and the consensus verifier both call this single function.