docs: sync spec with approved plan (--server arg detection)

docs/superpowers/specs/2026-04-14-tavern-lobby-design.md

@@ -51,8 +51,10 @@ SceneManager.transition_to("refectory")
 
 ```gdscript
 func _ready() -> void:
-    var is_server := OS.has_feature("dedicated_server") \
-        or "--server" in OS.get_cmdline_user_args()
+    # Check both arg lists: user args (after --) and engine args (before --)
+    # so --server works with or without the -- separator.
+    var args := OS.get_cmdline_args() + OS.get_cmdline_user_args()
+    var is_server := OS.has_feature("dedicated_server") or "--server" in args
     if not is_server:
         transition_to("tavern")
 ```