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.json

@@ -30,7 +30,8 @@
     "mockery/mockery": "^2.0",
     "laravel/pint": "^1.21",
     "pestphp/pest": "^4.7",
-    "larapack/dd": "dev-master"
+    "larapack/dd": "dev-master",
+    "phpstan/phpstan": "2.2.x-dev"
   },
   "autoload": {
     "psr-4": {