Track merchant payout address history

[awyoo]

Summary

• add persistent merchant payout address history keyed by merchant id
• record previous payout addresses only when the payout address actually changes
• emit MERCHANT/PAYOUT_UPDATED with (merchant_id, old_address, new_address) and add get_payout_history
• cover first-time set, ordered history retrieval, and unchanged-address updates

/claim #279

Payout address: 0xe80506A1431B3B4aAca8B260838481deBbdF75Ab

Verification

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