feat: add Cart::coupon(), Cart::tax(), Cart::shipping()

Proxy methods that accept first-class Coupon, TaxRule, and ShippingRate
objects and delegate to Cart::condition() via toCondition().

tests/Unit/CartFirstClassObjectsTest.php

@@ -0,0 +1,100 @@
+<?php
+
+declare(strict_types=1);
+
+use Wearepixel\Cart\Cart;
+use Wearepixel\Cart\CartCondition;
+use Wearepixel\Cart\Coupons\Coupon;
+use Wearepixel\Cart\Drivers\SessionDriver;
+use Wearepixel\Cart\Exceptions\InvalidCouponException;
+use Wearepixel\Cart\Shipping\ShippingRate;
+use Wearepixel\Cart\Tax\TaxRule;
+use Wearepixel\Cart\Tests\Helpers\SessionMock;
+
+function makeCart(): Cart
+{
+    $events = Mockery::mock('Illuminate\Contracts\Events\Dispatcher');
+    $events->shouldReceive('dispatch');
+
+    return new Cart(
+        new SessionDriver(new SessionMock, 'TESTKEY'),
+        $events,
+        'cart',
+        require __DIR__ . '/../Helpers/ConfigMock.php',
+    );
+}
+
+class TestCoupon extends Coupon
+{
+    protected string $code = 'SAVE10';
+    protected string $value = '-10%';
+    public function isValid(): bool { return true; }
+}
+
+class InvalidTestCoupon extends Coupon
+{
+    protected string $code = 'EXPIRED';
+    protected string $value = '-10%';
+    public function isValid(): bool { return false; }
+}
+
+class TestTax extends TaxRule
+{
+    protected string $name = 'GST';
+    protected string $value = '10%';
+    public function isApplicable(): bool { return true; }
+}
+
+class TestShipping extends ShippingRate
+{
+    protected string $name = 'Flat Rate';
+    protected string $value = '+10';
+    public function isApplicable(): bool { return true; }
+}
+
+beforeEach(function () {
+    $this->cart = makeCart();
+    $this->cart->add(1, 'Widget', 100.00, 1);
+});
+
+afterEach(fn() => Mockery::close());
+
+describe('Cart first-class objects', function () {
+    test('Cart::coupon() applies condition from a Coupon instance', function () {
+        $this->cart->coupon(new TestCoupon);
+
+        expect($this->cart->getCondition('SAVE10'))->toBeInstanceOf(CartCondition::class);
+        expect($this->cart->getSubTotal())->toEqual(90.00);
+    });
+
+    test('Cart::coupon() throws when coupon is invalid', function () {
+        expect(fn() => $this->cart->coupon(new InvalidTestCoupon))
+            ->toThrow(InvalidCouponException::class);
+    });
+
+    test('Cart::tax() applies condition from a TaxRule instance', function () {
+        $this->cart->tax(new TestTax);
+
+        expect($this->cart->getCondition('GST'))->toBeInstanceOf(CartCondition::class);
+        expect($this->cart->getSubTotal())->toEqual(110.00);
+    });
+
+    test('Cart::shipping() applies condition from a ShippingRate instance', function () {
+        $this->cart->shipping(new TestShipping);
+
+        expect($this->cart->getCondition('Flat Rate'))->toBeInstanceOf(CartCondition::class);
+        expect($this->cart->getTotal())->toEqual(110.00);
+    });
+
+    test('raw Cart::condition() still works alongside first-class objects', function () {
+        $this->cart->coupon(new TestCoupon);
+        $this->cart->condition(new CartCondition([
+            'name'   => 'Manual Discount',
+            'type'   => 'discount',
+            'value'  => '-5',
+            'target' => 'subtotal',
+        ]));
+
+        expect($this->cart->getConditions()->count())->toBe(2);
+    });
+});