Gate `Initialize` so it can't be frontrun

[kaze-cow]

Claude recommended this code so that we can gate Initialize using the upgrade authority of the program:

use pinocchio::{
    account_info::AccountInfo,
    program_error::ProgramError,
    pubkey::Pubkey,
    ProgramResult,
};

const BPF_LOADER_UPGRADEABLE_ID: Pubkey =
    pinocchio_pubkey::pubkey!("BPFLoaderUpgradeab1e11111111111111111111111");

/// Assert `authority` is the upgrade authority of THIS program.
/// Expects the program's own executable account, its ProgramData account,
/// and the claimed authority (signer).
fn assert_upgrade_authority(
    program: &AccountInfo,
    program_data: &AccountInfo,
    authority: &AccountInfo,
) -> ProgramResult {
    // 1. Signer actually signed.
    if !authority.is_signer() {
        return Err(ProgramError::MissingRequiredSignature);
    }

    // 2. `program` is our own program account.
    if program.key() != &crate::ID {
        return Err(ProgramError::IncorrectProgramId);
    }

    // 3. Both loader accounts owned by BPFLoaderUpgradeable.
    if !program.is_owned_by(&BPF_LOADER_UPGRADEABLE_ID)
        || !program_data.is_owned_by(&BPF_LOADER_UPGRADEABLE_ID)
    {
        return Err(ProgramError::InvalidAccountOwner);
    }

    // 4. Program account must point at the ProgramData we were handed.
    //    Layout: [u32 discriminant = 2][programdata_address: Pubkey(32)]
    {
        let data = program.try_borrow_data()?;
        if data.len() < 36 || data[0..4] != [2, 0, 0, 0] {
            return Err(ProgramError::InvalidAccountData);
        }
        if &data[4..36] != program_data.key().as_ref() {
            return Err(ProgramError::InvalidAccountData);
        }
    }

    // 5. ProgramData: upgrade authority must be Some(authority).
    //    Layout: [u32 discriminant = 3][slot: u64][Option<Pubkey>]
    //            option tag @ 12, pubkey @ 13..45
    {
        let data = program_data.try_borrow_data()?;
        if data.len() < 45 || data[0..4] != [3, 0, 0, 0] {
            return Err(ProgramError::InvalidAccountData);
        }
        if data[12] != 1 {
            // None => program is immutable, nobody qualifies
            return Err(ProgramError::InvalidAccountData);
        }
        if &data[13..45] != authority.key().as_ref() {
            return Err(ProgramError::InvalidAccountData);
        }
    }

    Ok(())
}