Register append! and dict-set! as proper primitives

Previously these mutating operations were internal helpers in the JS
bootstrapper but not declared in primitives.sx or registered in the
Python evaluator. Now properly specced and available in both hosts.

Removes mock injections from cache tests — they use real primitives.

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

shared/sx/ref/primitives.sx

@@ -384,6 +384,11 @@
   :returns "list"
   :doc "Append x to end of coll (returns new list).")
 
+(define-primitive "append!"
+  :params (coll x)
+  :returns "list"
+  :doc "Mutate coll by appending x in-place. Returns coll.")
+
 (define-primitive "chunk-every"
   :params (coll n)
   :returns "list"
@@ -426,6 +431,11 @@
   :returns "dict"
   :doc "Return new dict with keys removed.")
 
+(define-primitive "dict-set!"
+  :params (d key val)
+  :returns "any"
+  :doc "Mutate dict d by setting key to val in-place. Returns val.")
+
 (define-primitive "into"
   :params (target coll)
   :returns "any"