Wallet management for Haima.
Provides secp256k1 keypair generation, EVM-compatible address derivation, encrypted private key storage, and signing operations.
The wallet is local-first: private keys are encrypted with ChaCha20-Poly1305
and stored as Lago blobs. The abstraction layer supports future MPC wallet
backends (e.g., Coinbase CDP MPC) through the WalletBackend trait.