Enforce ASCII-only constraint per RFC 5322 specification

RFC 5322 (Internet Message Format) is strictly an ASCII-only specification.
This change ensures compliance by validating that email addresses contain only
ASCII characters.

Changes:
- Add ValidationError.nonASCIICharacters case with error description
- Validate ASCII-only in LocalPart.init using String.asciiBytes from INCITS_4_1986
- Replace manual character classification with INCITS_4_1986 utilities
  (isASCIILetter, isASCIIDigit, isASCIIWhitespace) in String.swift and [UInt8].swift
- Add import INCITS_4_1986 to String.swift

This prevents non-ASCII characters from being accepted in RFC 5322 contexts,
ensuring compliance with the specification and preventing potential issues with
email systems that expect ASCII-only message format.

Author: coenttb

Sources/RFC_5322/RFC_5322.EmailAddress.swift

@@ -110,6 +110,11 @@ extension RFC_5322.EmailAddress {
 
         /// Initialize with a string
         public init(_ string: String) throws {
+            // RFC 5322 is ASCII-only - validate before processing
+            guard string.asciiBytes != nil else {
+                throw ValidationError.nonASCIICharacters
+            }
+
             // Check overall length first
             guard string.count <= Limits.maxLength else {
                 throw ValidationError.localPartTooLong(string.count)
@@ -177,7 +182,7 @@ extension RFC_5322.EmailAddress {
 
 extension RFC_5322.EmailAddress {
     /// Just the email address part without display name
-    public var addressValue: String {
+    public var address: String {
         "\(localPart)@\(domain.name)"
     }
 }
@@ -191,6 +196,26 @@ extension RFC_5322.EmailAddress {
         case localPartTooLong(_ length: Int)
         case consecutiveDots
         case leadingOrTrailingDot
+        case nonASCIICharacters
+
+        public var errorDescription: String? {
+            switch self {
+            case .missingAtSign:
+                return "Email address must contain @"
+            case .invalidDotAtom:
+                return "Invalid local-part format (before @)"
+            case .invalidQuotedString:
+                return "Invalid quoted string format in local-part"
+            case .localPartTooLong(let length):
+                return "Local-part length \(length) exceeds maximum of \(Limits.maxLength)"
+            case .consecutiveDots:
+                return "Local-part cannot contain consecutive dots"
+            case .leadingOrTrailingDot:
+                return "Local-part cannot start or end with a dot"
+            case .nonASCIICharacters:
+                return "RFC 5322 email addresses must contain only ASCII characters"
+            }
+        }
     }
 }
 

Sources/RFC_5322/String.swift

@@ -5,6 +5,8 @@
 //  Type conversions for RFC 5322 Message
 //
 
+import INCITS_4_1986
+
 extension String {
     /// Creates RFC 5322 message string from a Message
     ///
@@ -28,7 +30,7 @@ extension String {
         if let name = emailAddress.displayName {
             // Quote the display name if it contains special characters or non-ASCII
             let needsQuoting = name.contains(where: {
-                !$0.isLetter && !$0.isNumber && !$0.isWhitespace || $0.asciiValue == nil
+                !$0.isASCIILetter && !$0.isASCIIDigit && !$0.isASCIIWhitespace || $0.asciiValue == nil
             })
             let quotedName = needsQuoting ? "\"\(name)\"" : name
             self = "\(quotedName) <\(emailAddress.localPart)@\(emailAddress.domain.name)>"  // Exactly one space before angle bracket

Sources/RFC_5322/[UInt8].swift

@@ -35,7 +35,7 @@ extension [UInt8] {
         if let displayName = emailAddress.displayName {
             // Check if quoting is needed
             let needsQuoting = displayName.contains(where: {
-                !$0.isLetter && !$0.isNumber && !$0.isWhitespace || $0.asciiValue == nil
+                !$0.isASCIILetter && !$0.isASCIIDigit && !$0.isASCIIWhitespace || $0.asciiValue == nil
             })
 
             if needsQuoting {