refactor(vault): remove irpc actor dispatch — direct method calls on VaultServiceHandle (ADR-025)

Drop the irpc-based actor dispatch path from alknet-vault and convert to
direct method calls on VaultServiceHandle (drift item #4, ADR-025).

Removed:
- VaultProtocol enum with #[rpc_requests] derive from protocol.rs
- VaultServiceActor (mpsc + oneshot dispatch loop) from service.rs
- VaultService wrapper struct (only the handle is needed)
- Client<VaultProtocol> usage
- irpc, irpc-derive, tokio from [dependencies]
- postcard from [dev-dependencies]
- VaultMessage/VaultProtocol/VaultServiceActor re-exports from lib.rs
- Serialize/Deserialize derives from VaultServiceError
- postcard round-trip tests from protocol.rs
- actor tokio::test tests from service.rs

The vault now has zero async runtime dependency and zero RPC framework
dependency — it is local-only by construction. VaultServiceHandle is the
sole API: Arc<std::sync::RwLock<VaultServiceInner>> with synchronous
methods. lib.rs re-exports match the vault README Public API section.

Also fixes pre-existing clippy field_reassign_with_default warnings in
cache.rs tests so cargo clippy -- -D warnings passes.

crates/alknet-vault/src/lib.rs

@@ -25,8 +25,8 @@
 //! - [`mnemonic`] — BIP39 mnemonic generation, validation, and seed derivation
 //! - [`derivation`] — SLIP-0010 Ed25519 HD key derivation and path constants
 //! - [`encryption`] — AES-256-GCM encrypt/decrypt and `EncryptedData` type
-//! - [`protocol`] — `VaultProtocol` irpc message enum, `DerivedKey`, `KeyType`
-//! - [`service`] — `VaultService` implementation with Unlock/Lock lifecycle
+//! - [`protocol`] — `DerivedKey` and `KeyType` (return types from vault methods)
+//! - [`service`] — `VaultServiceHandle` runtime API with Unlock/Lock lifecycle
 //! - [`ethereum`] — BIP-0032 secp256k1 HD key derivation (behind `secp256k1` feature)
 
 pub mod cache;
@@ -42,7 +42,8 @@ pub mod ethereum;
 // Re-export primary public API
 pub use cache::CacheConfig;
 pub use derivation::{DerivationError, ExtendedPrivKey, PATHS};
-pub use encryption::{EncryptedData, EncryptionError};
+pub use encryption::CURRENT_KEY_VERSION;
+pub use encryption::{EncryptedData, EncryptionError, EncryptionKey};
 pub use mnemonic::{Language, Mnemonic, Seed};
-pub use protocol::{DerivedKey, KeyType, VaultMessage, VaultProtocol};
-pub use service::{VaultService, VaultServiceActor, VaultServiceError, VaultServiceHandle};
+pub use protocol::{DerivedKey, KeyType};
+pub use service::{VaultServiceError, VaultServiceHandle};