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/tests/test_page_data.py

@@ -305,16 +305,6 @@ class TestDataCache:
         self._time = current_time_ms
         env["now-ms"] = lambda: self._time
 
-        # Mutating primitives needed by cache (available in JS, not bare Python)
-        def _dict_set(d, k, v):
-            d[k] = v
-            return v
-        def _append_b(lst, item):
-            lst.append(item)
-            return lst
-        env["dict-set!"] = _dict_set
-        env["append!"] = _append_b
-
         # Define the cache functions from orchestration.sx
         cache_src = """
         (define _page-data-cache (dict))