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is superseded by the ~tw component system in tw.sx.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

sx/sxc/init-client.sx.txt

@@ -20,9 +20,9 @@
     (log-info (str "cssx flush: running, rules:" (len (collected "cssx"))))
     (let ((rules (collected "cssx")))
       (when (not (empty? rules))
-        (let ((style (or (dom-query "[data-cssx]")
+        (let ((style (or (dom-query "[data-sx-css]")
                          (let ((s (dom-create-element "style" nil)))
-                           (dom-set-attr s "data-cssx" "")
+                           (dom-set-attr s "data-sx-css" "")
                            (dom-append-to-head s)
                            s))))
           (dom-set-prop style "textContent"