Implement VaultSigner: HKDF key derivation + Ed25519 signing #30
Description
Implement VaultSigner as a TransactionSigner:
- Deterministically derive Ed25519 keys for Solana using HKDF, with domain separation (info = "altude:vault:solana::").
- Use these keys for transaction and message signing.
- Integrate with the Vault module for key storage, unlock, and signing flows.