Remove incomplete concurrent CEK impl — spec only in plan

The concurrent CEK code (channels, spawn, fork-join) was incomplete
and untested. The full spec is in the foundations plan. Implementation
starts with phase 4a (Web Worker spawn) when ready.

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

shared/static/scripts/sx-browser.js

@@ -14,7 +14,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-15T00:27:14Z";
+  var SX_VERSION = "2026-03-15T00:53:02Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }