chore(phpstan): raise analyser memory limit and record resolved findings

Add --memory-limit=512M to the phpstan composer script — the default
128M crashed the level-8 run under parallel workers. Update
PHP-Stan-Errors.md to note all 22 catalogued findings are now resolved.

composer.json

@@ -41,5 +41,8 @@
             "allow-contrib": false,
             "require": "7.4.*"
         }
+    },
+    "scripts": {
+        "phpstan": "phpstan analyse --memory-limit=512M"
     }
 }