Phase 2: Move core spec files to spec/ and spec/tests/

git mv eval.sx, parser.sx, primitives.sx, render.sx, cek.sx, frames.sx,
continuations.sx, callcc.sx, types.sx, special-forms.sx → spec/
Tests → spec/tests/
Both bootstrappers verified — find files via spec/ → web/ → shared/sx/ref/

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 = "2026-03-15T00:53:02Z";
+  var SX_VERSION = "2026-03-15T02:11:58Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }