Release v0.1.0

Added

• Initial release of the payment processing smart contract for Soroban / Stellar.
• Merchant registration with optional whitelist mode and admin pre-approval.
• Signature-verified single payments (process_payment_with_signature).
• Multi-signature escrow payments (initiate_multisig_payment, sign_multisig_payment, execute_multisig_payment).
• Full refund workflow: initiate → approve/reject → execute.
• Paginated, filtered, and sorted payment history for merchants and payers.
• Global payment statistics with optional date-range filtering.
• Admin multi-sig threshold model (set_admin, upgrade).
• Configurable cleanup period and multi-sig expiry defaults.
• Per-environment TOML configuration files (config/local.toml, config/testnet.toml, config/mainnet.toml).
• Instance storage TTL extension on every invocation and public bump_instance_ttl function.
• Full /// rustdoc coverage on all public contract functions.