Verification

• Existence. The PreparedTx PDA must exist for the specified nonce.
• Replay protection. An ExecutedFlag for the same nonce must not already exist.
• Expiry. The nonce must still be within its validity window.

Fund movements

• Recipient payment. The pool pays publicAmount - fee to the external recipient.
• Relayer payment. The pool pays the percentage fee plus gas buffer to the relayer.

Cleanup

• Execution marker. An ExecutedFlag PDA is created to mark the nonce as consumed.
• PreparedTx closure. The PreparedTx PDA is closed and rent returned.