Move stdlib out of spec — clean spec/library boundary

spec/ now contains only the language definition (5 files):
  evaluator.sx, parser.sx, primitives.sx, render.sx, special-forms.sx

lib/ contains code written IN the language (8 files):
  stdlib.sx, types.sx, freeze.sx, content.sx,
  bytecode.sx, compiler.sx, vm.sx, callcc.sx

Test files follow source: spec/tests/ for core language tests,
lib/tests/ for library tests (continuations, freeze, types, vm).

Updated all consumers:
- JS/Python/OCaml bootstrappers: added lib/ to source search paths
- OCaml bridge: spec_dir for parser/render, lib_dir for compiler/freeze
- JS test runner: scans spec/tests/ (always) + lib/tests/ (--full)
- OCaml test runner: scans spec/tests/, lib tests via explicit request
- Docker dev mounts: added ./lib:/app/lib:ro

Tests: 1041 JS standard, 1322 JS full, 1101 OCaml — all pass

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 = "2026-03-24T22:31:01Z";
+  var SX_VERSION = "2026-03-24T22:56:21Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }