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Type Alias Callback signature for user-owned external-wallet signers. 
typedef CrossmintExternalWalletSignCallback = Future<String> Function(String payload)
Receives the payload to sign (hex-encoded for EVM, base58-encoded for Solana, base64-encoded transaction blob for Stellar) and returns the user-produced signature in the same encoding. Implementations typically delegate to a wallet SDK like WalletConnect, MetaMask Flask, or a platform-native wallet adapter.