Remove old CSSX system — ~tw is the sole CSS engine

Phase 1 Step 1 of the architecture roadmap. The old cssx.sx
(cssx-resolve, cssx-process-token, cssx-template, old tw function)
is superseded by the ~tw component system in tw.sx.

- Delete shared/sx/templates/cssx.sx
- Remove cssx.sx from all load lists (sx_server.ml, run_tests.ml,
  mcp_tree.ml, compile-modules.js, bundle.sh, sx-build-all.sh)
- Replace (tw "tokens") inline style calls with (~tw :tokens "tokens")
  in layouts.sx and not-found.sx
- Remove _css-hash / init-css-tracking / SX-Css header plumbing
  (dead code — ~tw/flush + flush-collected-styles handle CSS now)
- Remove sx-css-classes param and meta tag from shell template
- Update stale data-cssx references to data-sx-css in tests

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

hosts/ocaml/browser/compile-modules.js

@@ -37,7 +37,7 @@ const FILES = [
   'render.sx', 'core-signals.sx', 'signals.sx', 'deps.sx', 'router.sx',
   'page-helpers.sx', 'freeze.sx', 'bytecode.sx', 'compiler.sx', 'vm.sx',
   'dom.sx', 'browser.sx', 'adapter-html.sx', 'adapter-sx.sx', 'adapter-dom.sx',
-  'cssx.sx', 'tw-layout.sx', 'tw-type.sx', 'tw.sx',
+  'tw-layout.sx', 'tw-type.sx', 'tw.sx',
   'boot-helpers.sx', 'hypersx.sx', 'harness.sx', 'harness-reactive.sx',
   'harness-web.sx', 'engine.sx', 'orchestration.sx', 'boot.sx',
 ];