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/cache.rs

@@ -231,8 +231,10 @@ mod tests {
 
     #[test]
     fn test_cache_expired_entry_evicted_on_access() {
-        let mut config = CacheConfig::default();
-        config.ttl = Duration::from_millis(1);
+        let config = CacheConfig {
+            ttl: Duration::from_millis(1),
+            ..Default::default()
+        };
 
         let mut cache = KeyCache::new(config);
         cache.insert("m/74'/0'/0'/0'", make_cached_key(KeyType::Ed25519));
@@ -245,8 +247,10 @@ mod tests {
 
     #[test]
     fn test_cache_lru_eviction() {
-        let mut config = CacheConfig::default();
-        config.max_entries = 3;
+        let config = CacheConfig {
+            max_entries: 3,
+            ..Default::default()
+        };
 
         let mut cache = KeyCache::new(config);
 
@@ -267,8 +271,10 @@ mod tests {
 
     #[test]
     fn test_cache_lru_access_reorders() {
-        let mut config = CacheConfig::default();
-        config.max_entries = 3;
+        let config = CacheConfig {
+            max_entries: 3,
+            ..Default::default()
+        };
 
         let mut cache = KeyCache::new(config);
 
@@ -303,8 +309,10 @@ mod tests {
 
     #[test]
     fn test_evict_expired_removes_only_expired() {
-        let mut config = CacheConfig::default();
-        config.ttl = Duration::from_millis(10);
+        let config = CacheConfig {
+            ttl: Duration::from_millis(10),
+            ..Default::default()
+        };
 
         let mut cache = KeyCache::new(config);
         cache.insert("path1", make_cached_key(KeyType::Ed25519));