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>

hosts/python/bootstrap.py

@@ -1448,6 +1448,7 @@ def compile_ref_to_py(
     _project = os.path.abspath(os.path.join(ref_dir, "..", "..", ".."))
     _source_dirs = [
         os.path.join(_project, "spec"),
+        os.path.join(_project, "lib"),
         os.path.join(_project, "web"),
         ref_dir,
     ]