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is superseded by the ~tw component system in tw.sx.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/bin/mcp_tree.ml

@@ -609,7 +609,7 @@ let rec handle_tool name args =
       "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";
     ] in