Content-addressed computation: freeze → hash → CID → thaw

Hash frozen SX to a content identifier (djb2 → hex). Same state
always produces the same CID. Store by CID, retrieve by CID.

- content-hash: djb2 hash of SX text → hex string
- content-put/get: in-memory content store
- freeze-to-cid: freeze scope → store → return CID
- thaw-from-cid: look up CID → thaw signals
- char-code-at / to-hex primitives for both platforms
- Live demo: counter + name widget, content-address button,
  CID display, restore from CID input, CID history

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

shared/sx/ref/platform_js.py

@@ -1532,6 +1532,10 @@ CEK_FIXUPS_JS = '''
   PRIMITIVES["is-html-tag?"] = function(n) { return HTML_TAGS.indexOf(n) >= 0; };
   PRIMITIVES["make-env"] = function() { return merge(componentEnv, PRIMITIVES); };
 
+  // String/number utilities for content addressing
+  PRIMITIVES["char-code-at"] = function(s, i) { return s.charCodeAt(i); };
+  PRIMITIVES["to-hex"] = function(n) { return (n >>> 0).toString(16); };
+
   // localStorage — defined here (before boot) so islands can use at hydration
   PRIMITIVES["local-storage-get"] = function(key) {
     try { var v = localStorage.getItem(key); return v === null ? NIL : v; }