Add Content Addressing page under CEK Machine

Dedicated page documenting and demonstrating content-addressed
computation. How it works, why it matters, the path to IPFS.

Live demo: counter + name widget with CID generation, history,
and restore-from-CID input.

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

shared/sx/ref/platform_js.py

@@ -1534,7 +1534,9 @@ CEK_FIXUPS_JS = '''
 
   // String/number utilities for content addressing
   PRIMITIVES["char-code-at"] = function(s, i) { return s.charCodeAt(i); };
+  var charCodeAt = PRIMITIVES["char-code-at"];
   PRIMITIVES["to-hex"] = function(n) { return (n >>> 0).toString(16); };
+  var toHex = PRIMITIVES["to-hex"];
 
   // localStorage — defined here (before boot) so islands can use at hydration
   PRIMITIVES["local-storage-get"] = function(key) {