Fix examples.sx: paren balance + dict eval crash at startup

1. Extra closing paren in ex-tabs handler
2. tab-content dict values contained (div ...) HTML tags which crash
   during register_components since HTML primitives aren't in env

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/primitives.sx

@@ -70,6 +70,17 @@
   :doc "Modulo a % b."
   :body (native-mod a b))
 
+(define-primitive "random-int"
+  :params ((low :as number) (high :as number))
+  :returns "number"
+  :doc "Random integer in [low, high] inclusive."
+  :body (native-random-int low high))
+
+(define-primitive "json-encode"
+  :params (value)
+  :returns "string"
+  :doc "Encode value as JSON string with indentation.")
+
 (define-primitive "sqrt"
   :params ((x :as number))
   :returns "number"