Function build_unshield_transition 

pub fn build_unshield_transition<P: OrchardProver>(
    spends: Vec<SpendableNote>,
    output_address: PlatformAddress,
    unshield_amount: u64,
    change_address: &OrchardAddress,
    fvk: &FullViewingKey,
    ask: &SpendAuthorizingKey,
    anchor: Anchor,
    prover: &P,
    memo: [u8; 36],
    platform_version: &PlatformVersion,
) -> Result<(StateTransition, Credits), ProtocolError>

Builds an Unshield state transition (shielded pool -> platform address).

Spends existing notes and sends part of the value to a transparent platform address. The shielded fee is deducted from the spent notes. Any remaining value is returned to the shielded change_address; the change note is encrypted with the sender’s External-scope OVK (derived from fvk) so the wallet can recover it — including the structured memo — via OVK recovery.

Parameters

• spends - Notes to spend with their Merkle paths
• output_address - Platform address to receive the unshielded funds
• unshield_amount - Amount to unshield to the platform address
• change_address - Orchard address for change output
• fvk - Full viewing key for spend authorization
• ask - Spend authorizing key for RedPallas signatures
• anchor - Sinsemilla root of the note commitment tree (Orchard Anchor)
• prover - Orchard prover (holds the Halo 2 proving key)
• memo - 36-byte structured memo for the change output (4-byte type tag + 32-byte payload)
• platform_version - Protocol version

The fee is not a parameter: consensus always charges exactly compute_shielded_unshield_fee (the base shielded minimum fee PLUS the flat storage cost of the single AddBalanceToAddress write this transition performs crediting the net to the output address) and ignores any surplus. Returns the built transition together with the fee (in credits) that was applied.