Improved type safety, local auth support, and better error handling

Sources/KeychainKit/Enums/KeychainError.swift

@@ -0,0 +1,15 @@
+import Foundation
+
+/// Errors that can occur during Keychain operations.
+public enum KeychainError: Error {
+    /// Authentication failed, e.g., due to biometric or passcode denial.
+    case authenticationFailed
+    /// No item found matching the query.
+    case itemNotFound
+    /// Unexpected or corrupted data found in Keychain item.
+    case unexpectedData
+    /// An unexpected OSStatus error code returned by Keychain API.
+    case unexpectedCode(OSStatus)
+    /// A generic unexpected error, with optional underlying error info.
+    case unexpectedError(Error?)
+}