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is superseded by the ~tw component system in tw.sx.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>

scripts/sx-build-all.sh

@@ -26,7 +26,7 @@ for f in 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 \
          boot-helpers.sx hypersx.sx harness-reactive.sx harness-web.sx \
-         engine.sx orchestration.sx boot.sx cssx.sx \
+         engine.sx orchestration.sx boot.sx \
          tw-layout.sx tw-type.sx tw.sx; do
   if [ -f "shared/static/wasm/sx/$f" ]; then
     cp "shared/static/wasm/sx/$f" "hosts/ocaml/browser/dist/sx/"