Assert DataKey storage round trips

[awyoo]

Summary

• replace the empty test_datakey_discriminant_stability body with real persistent storage checks
• cover DataKey::Payment, DataKey::Refund, and DataKey::MerchantPayments
• document why #[contracttype] enum variant order is storage compatibility surface

/claim #338

Payout address: 0xe80506A1431B3B4aAca8B260838481deBbdF75Ab

Verification

• git diff --check
• Not run: cargo test -p fluxapay test_datakey_discriminant_stability -- --nocapture because this local environment does not have cargo/rustc installed.