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is superseded by the ~tw component system in tw.sx.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

@@ -1657,7 +1657,6 @@ let http_render_page env path headers =
             Keyword "component-hash"; get_shell "component-hash";
             Keyword "pages-sx"; get_shell "pages-sx";
             Keyword "sx-css"; get_shell "sx-css";
-            Keyword "sx-css-classes"; get_shell "sx-css-classes";
             Keyword "asset-url"; get_shell "asset-url";
             Keyword "wasm-hash"; get_shell "wasm-hash";
             Keyword "platform-hash"; get_shell "platform-hash";
@@ -1771,7 +1770,6 @@ let http_inject_shell_statics env static_dir sx_sxc =
     Filename.dirname (Filename.dirname static_dir) in
   let templates_dir = project_dir ^ "/shared/sx/templates" in
   let client_libs = [
-    templates_dir ^ "/cssx.sx";
     templates_dir ^ "/tw-layout.sx";
     templates_dir ^ "/tw-type.sx";
     templates_dir ^ "/tw.sx";
@@ -1860,7 +1858,6 @@ let http_inject_shell_statics env static_dir sx_sxc =
     (List.length (String.split_on_char '\n' pages_sx));
   ignore (env_bind env "__shell-pages-sx" (String pages_sx));
   ignore (env_bind env "__shell-sx-css" (String sx_css));
-  ignore (env_bind env "__shell-sx-css-classes" (String ""));
   ignore (env_bind env "__shell-asset-url" (String "/static"));
   ignore (env_bind env "__shell-wasm-hash" (String wasm_hash));
   ignore (env_bind env "__shell-platform-hash" (String platform_hash));