fix: validate slot mode on first defslot call

Add assert to reject invalid mode keywords on first registration
instead of silently storing them and only crashing later in
slot-render's ecase. Valid modes: :stack, :replace, :single-winner.

org/slot.org

@@ -93,8 +93,8 @@ the first call and frozen for subsequent calls:
 
 The ~render-fn~ itself is returned so callers can use it inline.
 
-The mode parameter is accepted but only respected on the first
-registration for a slot. This prevents a later registration from
+The mode parameter is validated on first call via ~assert~ and then
+frozen for subsequent calls. This prevents a later registration from
 changing the slot's semantics out from under earlier registrations.
 
 #+BEGIN_SRC lisp :tangle ../src/components/slot.lisp
@@ -102,10 +102,14 @@ changing the slot's semantics out from under earlier registrations.
   (let* ((key (string name))
          (slot (gethash key *slots*)))
     (if (null slot)
-        ;; First registration — set mode and create entry
-        (setf (gethash key *slots*)
-              (list :mode mode
-                    :entries (list (cons order render-fn))))
+        ;; First registration — validate and set mode, create entry
+        (progn
+          (assert (member mode '(:stack :replace :single-winner)) ()
+                  "Invalid slot mode: ~S (use :stack, :replace, or :single-winner)"
+                  mode)
+          (setf (gethash key *slots*)
+                (list :mode mode
+                      :entries (list (cons order render-fn)))))
         ;; Existing slot — respect frozen mode
         (let ((entries (getf slot :entries)))
           (ecase (getf slot :mode)