Function build_shield_transition 

pub async fn build_shield_transition<S: Signer<PlatformAddress>, P: OrchardProver>(
    recipient: &OrchardAddress,
    shield_amount: u64,
    inputs: BTreeMap<PlatformAddress, (AddressNonce, Credits)>,
    fee_strategy: AddressFundsFeeStrategy,
    signer: &S,
    user_fee_increase: UserFeeIncrease,
    prover: &P,
    memo: [u8; 36],
    sender_ovk: Option<OutgoingViewingKey>,
    platform_version: &PlatformVersion,
) -> Result<StateTransition, ProtocolError>

Builds a Shield state transition (transparent platform addresses -> shielded pool).

Constructs an output-only Orchard bundle (no spends), proves it, signs the transparent input witnesses, and returns a ready-to-broadcast StateTransition.

Parameters

• recipient - Orchard address to receive the shielded note
• shield_amount - Amount of credits to shield
• inputs - Platform address inputs with their nonces and balances
• fee_strategy - How to deduct fees from the transparent inputs
• signer - Signs each input address witness (ECDSA)
• user_fee_increase - Fee multiplier (0 = 100% base fee)
• prover - Orchard prover (holds the Halo 2 proving key; cache with OnceLock — ~30s to build)
• memo - 36-byte structured memo for the recipient (4-byte type tag + 32-byte payload)
• sender_ovk - The sender’s outgoing viewing key (External scope). With Some, the recipient output’s out_ciphertext is encrypted under it so the sender can later recover the sent note (recipient, value, memo) from chain data via OVK recovery — the Zcash outgoing-transaction-history convention. With None, a random outgoing cipher key is used and the sent note is unrecoverable by anyone.
• platform_version - Protocol version