Stable extension point for definition-form? — no monkey-patching

Replace the fragile pattern of capturing and wrapping definition-form?
with a mutable *definition-form-extensions* list in render.sx. Web
modules append names to this list instead of redefining the function.
Survives spec reloads without losing registrations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/render.sx

@@ -71,11 +71,16 @@
 ;; Shared utilities
 ;; --------------------------------------------------------------------------
 
+;; Extension point for definition forms — modules append names here.
+;; Survives spec reloads (no function wrapping needed).
+(define *definition-form-extensions* (list))
+
 (define definition-form? :effects []
   (fn ((name :as string))
     (or (= name "define") (= name "defcomp") (= name "defisland")
         (= name "defmacro") (= name "defstyle")
-        (= name "deftype") (= name "defeffect"))))
+        (= name "deftype") (= name "defeffect")
+        (contains? *definition-form-extensions* name))))
 
 
 (define parse-element-args :effects [render]