feat: implement enhanced KeyStore with Key trait integration

* Add comprehensive EnhancedKeyStore implementation with PKI-specific metadata
* Extend Key trait with as_any() method for safe type downcasting
* Support all key types: Ed25519, P256, RSA with PKCS#8 serialization
* Implement hybrid metadata persistence (cache + attribute storage)
* Add PKI-specific features: usage restrictions, validity periods, certificate associations
* Include comprehensive test coverage for all supported key algorithms
* Bridge gap between raw key storage and high-level PKI operations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: atlas-form

crates/capsula-key/src/key/curve25519.rs

@@ -201,6 +201,10 @@ impl Key for Curve25519 {
     fn capabilities(&self) -> KeyCapabilities {
         KeyCapabilities::SIGNING.union(KeyCapabilities::KEY_AGREEMENT)
     }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 impl KeySign for Curve25519 {

crates/capsula-key/src/key/mod.rs

@@ -53,6 +53,9 @@ pub trait Key: Send + Sync {
     fn fingerprint_hex(&self) -> String {
         hex::encode(self.fingerprint_sha256_spki())
     }
+
+    /// 获取Any trait object引用，用于安全的向下转换
+    fn as_any(&self) -> &dyn std::any::Any;
 }
 
 // ============================================================================

crates/capsula-key/src/key/p256.rs

@@ -119,6 +119,10 @@ impl Key for P256Key {
     fn capabilities(&self) -> KeyCapabilities {
         KeyCapabilities::SIGNING.union(KeyCapabilities::KEY_AGREEMENT)
     }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 // ============================================================================

crates/capsula-key/src/key/rsa.rs

@@ -116,6 +116,10 @@ impl Key for RsaKey {
     fn capabilities(&self) -> KeyCapabilities {
         KeyCapabilities::SIGNING.union(KeyCapabilities::ENCRYPTION)
     }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 // ============================================================================