Opt the select into base-select from its own stylesheet

Plan step 36. menu.css left Tailwind's unlayered half for
material.components, where its `appearance: base-select` on the select
ties select.css's `appearance: none` in layer and specificity and loses
on order: select.css's block always follows the menu.css it imports. The
customizable select would have fallen back to the native list. The
select's own opt-in now sits in select.css's @supports block; menu.css
keeps the picker's. A test pins where each is written.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/components/menu.css

@@ -320,7 +320,9 @@
     }
 
     @supports (appearance: base-select) {
-        select[data-md-field-control],
+        /* The select itself opts in from select.css, after its own `appearance: none`: both sit in
+           `material.components` with the same specificity, and select.css's block always comes
+           after this file, which it imports. */
         select[data-md-field-control]::picker(select) {
             appearance: base-select;
         }