feat: add toBeEmail expectation (#1735)

Adds `toBeEmail()` backed by `filter_var(FILTER_VALIDATE_EMAIL)`.
Mirrors the existing `toBeUrl()` pattern — no new dependencies, pure PHP.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Features/Expect/toBeEmail.php

@@ -0,0 +1,24 @@
+<?php
+
+use PHPUnit\Framework\ExpectationFailedException;
+
+test('pass', function () {
+    expect('user@example.com')->toBeEmail()
+        ->and('notanemail')->not->toBeEmail();
+});
+
+test('failures', function () {
+    expect('notanemail')->toBeEmail();
+})->throws(ExpectationFailedException::class);
+
+test('failures with custom message', function () {
+    expect('notanemail')->toBeEmail('oh no!');
+})->throws(ExpectationFailedException::class, 'oh no!');
+
+test('failures with default message', function () {
+    expect('notanemail')->toBeEmail();
+})->throws(ExpectationFailedException::class, 'Failed asserting that notanemail is an email address.');
+
+test('not failures', function () {
+    expect('user@example.com')->not->toBeEmail();
+})->throws(ExpectationFailedException::class);