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is superseded by the ~tw component system in tw.sx.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

shared/sx/templates/shell.sx

@@ -5,7 +5,6 @@
     (meta-html :as string?)
     (csrf :as string)
     (sx-css :as string?)
-    (sx-css-classes :as string?)
     (component-hash :as string?)
     (component-defs :as string?)
     (pages-sx :as string?)
@@ -37,7 +36,6 @@
           (when
             (not (empty? cssx-rules))
             (style :data-sx-css true (raw! (join "" cssx-rules)))))
-        (meta :name "sx-css-classes" :content (or sx-css-classes ""))
         (if
           (not (nil? inline-head-js))
           (when