Change CloudFrontSignerError from enum to struct

Uses a private enum code with static properties for better ABI stability
and future extensibility without breaking API changes.

Author: sebsto

Sources/SotoSignerCloudFront/CloudFrontSigner+Internal.swift

@@ -31,8 +31,10 @@ extension CloudFrontSigner {
     /// - `/` → `~`
     ///
     /// Note: This is NOT standard base64url (RFC 4648 §5) — CloudFront uses `~` for `/`
-    /// and `_` for padding `=`, which differs from base64url's `_` for `/` with stripped padding.
-    /// Therefore `base64EncodedString(options: .base64URLAlphabet)` cannot be used here.
+    /// and `_` for padding `=`, which differs from base64url's `_` for `/` with padding kept.
+    /// While `.base64URLAlphabet` (macOS 26.4+) could be used as a starting point with
+    /// two additional replacements (`_` → `~`, `=` → `_`), it offers no simplification
+    /// over the current approach and requires a newer deployment target.
     static func cloudFrontBase64Encode(_ data: Data) -> String {
         let base64 = data.base64EncodedString()
         var result = ""

Sources/SotoSignerCloudFront/CloudFrontSignerError.swift

@@ -13,11 +13,24 @@
 //===----------------------------------------------------------------------===//
 
 /// Errors that can occur during CloudFront signing operations
-public enum CloudFrontSignerError: Error, Sendable {
-    /// The provided PEM data could not be parsed as an RSA private key
-    case invalidPrivateKey
+public struct CloudFrontSignerError: Error, Sendable, Equatable {
+
+    private enum Code: String, Sendable {
+        case invalidPrivateKey
+        case invalidURL
+        case signingFailed
+    }
+
+    private let code: Code
+
+    private init(_ code: Code) {
+        self.code = code
+    }
+
+    /// The provided PEM/DER data could not be parsed as an RSA private key
+    public static let invalidPrivateKey = CloudFrontSignerError(.invalidPrivateKey)
     /// The URL string is not valid
-    case invalidURL
+    public static let invalidURL = CloudFrontSignerError(.invalidURL)
     /// The signing operation failed
-    case signingFailed
+    public static let signingFailed = CloudFrontSignerError(.signingFailed)
 }