email: validate with net/mail.ParseAddress

Replace the RFC 5322 mega-regexp with net/mail.ParseAddress, which is the
standard library's address parser and far easier to audit. A bare address
is required: inputs with a display name, angle brackets, a comment, or
trailing content are rejected, as is an address list.

Behaviour change: ParseAddress does not require the domain to contain a
dot, so "alice@localhost" now validates. Layer Match or a DNS lookup on
top if a stricter domain is needed.

Also rewrites FuzzEmail, which previously asserted that *every* input is
invalid, into checks that Email never returns an unexpected error type,
never panics, and gives a stable verdict.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

email_test.go

@@ -12,14 +12,26 @@ func ExampleEmail() {
 }
 
 func FuzzEmail(f *testing.F) {
-	want := ErrInvalidEmail
+	for _, seed := range []string{
+		"", "alice@example.com", "not an email", "a@b",
+		"Alice <alice@example.com>", "alice@example.com ",
+	} {
+		f.Add(seed)
+	}
 
 	f.Fuzz(func(t *testing.T, input string) {
-		got := Email(input)
+		err := Email(input)
 
-		if !errors.Is(got, want) {
-			t.Error("got", got)
-			t.Error("want", want)
+		// The only error Email ever returns is ErrInvalidEmail.
+		if err != nil && !errors.Is(err, ErrInvalidEmail) {
+			t.Fatalf("unexpected error for %q: %v", input, err)
+		}
+
+		// A verdict of "valid" must be stable on re-validation.
+		if err == nil {
+			if err2 := Email(input); err2 != nil {
+				t.Fatalf("inconsistent verdict for %q: first nil, then %v", input, err2)
+			}
 		}
 	})
 }
@@ -28,9 +40,15 @@ func TestEmail(t *testing.T) {
 	testCases := map[string]error{
 		"test@example.com":                      nil,
 		"firstname.lastname@some-website.co.uk": nil,
+		"alice+tag@example.com":                 nil,
+		"alice@localhost":                       nil, // ParseAddress does not require a dotted domain
 
-		"not an email":    ErrInvalidEmail,
-		"testexample.com": ErrInvalidEmail,
+		"not an email":              ErrInvalidEmail,
+		"testexample.com":           ErrInvalidEmail,
+		"Alice <alice@example.com>": ErrInvalidEmail, // display name
+		"<alice@example.com>":       ErrInvalidEmail, // angle brackets
+		"alice@example.com ":        ErrInvalidEmail, // trailing space
+		"a@b.com, c@d.com":          ErrInvalidEmail, // address list
 	}
 
 	for input, want := range testCases {