Fix TLD validation to require fully alphabetic TLD per RFC 1123

RFC 1123 Section 2.1 states "the highest-level component label will
be alphabetic". This updates validation to check that the entire TLD
consists only of letters, not just that it starts with a letter.

Author: coenttb

Sources/RFC 1123/RFC_1123.Domain.swift

@@ -89,11 +89,11 @@ extension RFC_1123.Domain: Hashable {
     }
 }
 
-extension RFC_1123.Domain: UInt8.ASCII.RawRepresentable {}
+extension RFC_1123.Domain: Binary.ASCII.RawRepresentable {}
 
 extension RFC_1123.Domain: CustomStringConvertible {}
 
-extension RFC_1123.Domain: UInt8.ASCII.Serializable {
+extension RFC_1123.Domain: Binary.ASCII.Serializable {
     public static func serialize<Buffer: RangeReplaceableCollection>(
         ascii domain: Self,
         into buffer: inout Buffer
@@ -185,11 +185,10 @@ extension RFC_1123.Domain: UInt8.ASCII.Serializable {
             }
         }
 
-        // RFC 1123 hostname-level constraint:
-        // "at least the highest-level component label will be alphabetic"
+        // RFC 1123 Section 2.1:
+        // "the highest-level component label will be alphabetic"
         if let tld = labels.last {
-            let firstByte = tld.rawValue.utf8.first!
-            guard firstByte.ascii.isLetter else {
+            guard tld.rawValue.utf8.allSatisfy({ $0.ascii.isLetter }) else {
                 throw Error.invalidTLD(tld.rawValue)
             }
         }
@@ -287,10 +286,10 @@ extension RFC_1123.Domain {
             throw Error.tooLong(name.count)
         }
 
-        // Validate TLD constraint
+        // RFC 1123 Section 2.1:
+        // "the highest-level component label will be alphabetic"
         if let tld = labels.last {
-            let firstByte = tld.rawValue.utf8.first!
-            guard firstByte.ascii.isLetter else {
+            guard tld.rawValue.utf8.allSatisfy({ $0.ascii.isLetter }) else {
                 throw Error.invalidTLD(tld.rawValue)
             }
         }

Tests/RFC 1123 Tests/RFC 1123 Tests.swift

@@ -43,11 +43,11 @@ struct `RFC 1123 Host Tests` {
     }
 
     @Test
-    func `Successfully creates host with TLD ending with number`() throws {
-        // RFC 1123 only requires TLD to START with a letter, not end with one
-        let host = try RFC_1123.Domain("example.com123")
-        #expect(host.name == "example.com123")
-        #expect(host.tld?.rawValue == "com123")
+    func `Fails with invalid TLD ending with number`() throws {
+        // RFC 1123 Section 2.1: "the highest-level component label will be alphabetic"
+        #expect(throws: RFC_1123.Domain.Error.invalidTLD("com123")) {
+            _ = try RFC_1123.Domain("example.com123")
+        }
     }
 
     @Test