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is superseded by the ~tw component system in tw.sx.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

@@ -1168,7 +1168,6 @@ let run_spec_tests env test_files =
   load_module "tw.sx" templates_dir;
   load_module "tw-layout.sx" templates_dir;
   load_module "tw-type.sx" templates_dir;
-  load_module "cssx.sx" templates_dir;
   (* SX docs site: components, handlers, demos *)
   let sx_comp_dir = Filename.concat project_dir "sx/sxc" in
   let sx_sx_dir = Filename.concat project_dir "sx/sx" in