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.go

@@ -1,19 +1,23 @@
 package validate
 
 import (
-	"regexp"
+	"net/mail"
 )
 
 var (
 	ErrInvalidEmail = NewError("invalid email address")
 )
 
-// Based on https://stackoverflow.com/a/201378
-var emailRegexp = regexp.MustCompile("^(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|\"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*\")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\\[(?:(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9]))\\.){3}(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\\])$")
-
-// Email validates an email address.
+// Email validates an email address using net/mail.ParseAddress.
+//
+// Only a bare address is accepted (alice@example.com). Anything with a
+// display name, angle brackets, a comment, or trailing content is rejected,
+// as is a list of addresses. Note that ParseAddress does not require the
+// domain to have a dot, so "alice@localhost" is considered valid; layer on
+// Match or a DNS check if you need to be stricter.
 func Email(value string) error {
-	if !emailRegexp.MatchString(value) {
+	addr, err := mail.ParseAddress(value)
+	if err != nil || addr.Name != "" || addr.Address != value {
 		return ErrInvalidEmail
 	}