feat: add PHPStan level 5 with zero errors

- Install phpstan/phpstan, add phpstan.neon at level 5 targeting PHP 8.3
- Add phpstan-bootstrap.php stubs for Laravel helper functions (app,
  config, config_path, app_path, session) unavailable in standalone packages
- Fix CartCondition::setOrder() - removed incorrect @return int (method is void)
- Fix CartCondition::getCalculatedValue() - removed incorrect @return mixed PHPDoc
- Fix Cart::condition() - remove redundant instanceof check the type system already guarantees
- Fix Cart::fireEvent() - remove array_values() on already-list literal array
- Fix ItemCollection - add @property annotations for magic __get properties
- Fix ItemCollection::getAssociatedModel() - correct @return bool to @return mixed
- Fix ItemCollection::getConditions() - remove dead is_array && !instanceof check
- Fix Validator - add @method stub so PHPStan resolves make() through __callStatic
- Add ignoreErrors for trait.unused (HasCart designed for external Livewire use)
- Add ignoreErrors for round() mode mismatch (PHPStan 2.x PHP 8.4 stubs bug)
- Add phpstan step to CI (PHP 8.3 only, matches phpVersion config)

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "36458db92bb67aee0a12a15f588349b4",
+    "content-hash": "56b40d3f17cc40f1b62c3f71a3601af7",
     "packages": [
         {
             "name": "brick/math",
@@ -6518,6 +6518,71 @@
             },
             "time": "2026-05-25T15:58:15+00:00"
         },
+        {
+            "name": "phpstan/phpstan",
+            "version": "2.2.x-dev",
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan/zipball/5c0bbadd35255710f7d2402cd6213a0bc34cfa34",
+                "reference": "5c0bbadd35255710f7d2402cd6213a0bc34cfa34",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.4|^8.0"
+            },
+            "conflict": {
+                "phpstan/phpstan-shim": "*"
+            },
+            "default-branch": true,
+            "bin": [
+                "phpstan",
+                "phpstan.phar"
+            ],
+            "type": "library",
+            "autoload": {
+                "files": [
+                    "bootstrap.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Ondřej Mirtes"
+                },
+                {
+                    "name": "Markus Staab"
+                },
+                {
+                    "name": "Vincent Langlet"
+                }
+            ],
+            "description": "PHPStan - PHP Static Analysis Tool",
+            "keywords": [
+                "dev",
+                "static analysis"
+            ],
+            "support": {
+                "docs": "https://phpstan.org/user-guide/getting-started",
+                "forum": "https://github.com/phpstan/phpstan/discussions",
+                "issues": "https://github.com/phpstan/phpstan/issues",
+                "security": "https://github.com/phpstan/phpstan/security/policy",
+                "source": "https://github.com/phpstan/phpstan-src"
+            },
+            "funding": [
+                {
+                    "url": "https://github.com/ondrejmirtes",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/phpstan",
+                    "type": "github"
+                }
+            ],
+            "time": "2026-06-05T16:36:10+00:00"
+        },
         {
             "name": "phpunit/php-code-coverage",
             "version": "12.5.x-dev",
@@ -8189,7 +8254,8 @@
     "aliases": [],
     "minimum-stability": "dev",
     "stability-flags": {
-        "larapack/dd": 20
+        "larapack/dd": 20,
+        "phpstan/phpstan": 20
     },
     "prefer-stable": false,
     "prefer-lowest": false,