refactor(vault): remove irpc actor dispatch — direct method calls on VaultServiceHandle (task: vault/irpc-removal)

ADR-025 / drift item #4: remove the irpc-based actor dispatch from the vault
crate. VaultServiceHandle (Arc<std::sync::RwLock<>>) is now the sole synchronous
API. Removed: VaultProtocol enum, VaultServiceActor, VaultService wrapper,
Client<VaultProtocol> usage, irpc/irpc-derive/tokio deps, postcard dev-dep,
Serialize/Deserialize on VaultServiceError. lib.rs re-exports match the vault
README Public API. The vault is now local-only by construction with zero async
runtime dependency.

Refs: docs/architecture/crates/vault/README.md drift #4
Implements: ADR-025

# Conflicts:
#	Cargo.lock

crates/alknet-vault/src/cache.rs

@@ -309,8 +309,10 @@ 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));
@@ -323,8 +325,10 @@ 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);
 
@@ -345,8 +349,10 @@ 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);
 
@@ -381,8 +387,10 @@ 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));