Phase 7e: isomorphic testing — cross-host Python/JS comparison

61 tests evaluate the same SX expressions on both Python (sx_ref.py)
and JS (sx-browser.js via Node.js), comparing output:
- 37 eval tests (arithmetic, strings, collections, logic, HO forms)
- 24 render tests (elements, attrs, void elements, components, escaping)

All pass — both hosts produce identical results from the same spec.

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

sx/sx/plans.sx

@@ -2046,7 +2046,21 @@
         (p "Service Worker intercepts /internal/data/ requests, serves from IndexedDB when offline, syncs when back online."))
 
       (~doc-subsection :title "7e. Isomorphic Testing"
-        (p "Evaluate same component on Python and JS, compare output. Extends existing test_sx_ref.py cross-evaluator comparison."))
+
+        (div :class "rounded border border-green-300 bg-green-50 p-3 mb-4"
+          (div :class "flex items-center gap-2 mb-1"
+            (span :class "inline-block px-2 py-0.5 rounded text-xs font-bold bg-green-600 text-white uppercase" "Complete"))
+          (p :class "text-green-800 text-sm" "Cross-host test suite: same SX expressions evaluated on Python (sx_ref.py) and JS (sx-browser.js via Node.js), HTML output compared."))
+
+        (p "61 isomorphic tests verify that Python and JS produce identical results:")
+        (ul :class "list-disc pl-5 text-stone-700 space-y-1"
+          (li "37 eval tests: arithmetic, comparison, strings, collections, logic, let/lambda, higher-order, dict, keywords, cond/case")
+          (li "24 render tests: elements, attributes, nesting, void elements, boolean attrs, conditionals, map, components, affinity, HTML escaping"))
+
+        (~doc-subsection :title "Files"
+          (ul :class "list-disc pl-5 text-stone-700 space-y-1 font-mono text-sm"
+            (li "shared/sx/tests/test_isomorphic.py — cross-host test suite")
+            (li "Run: " (code "python3 -m pytest shared/sx/tests/test_isomorphic.py -q")))))
 
       (~doc-subsection :title "7f. Universal Page Descriptor"
         (p "defpage is portable: server executes via execute_page(), client executes via route match → fetch data → eval content → render DOM. Same descriptor, different execution environment."))