CSSX rules from island SSR: flush collected rules via ~cssx/flush in shell

Added (~cssx/flush) to shell after sx-root div — picks up CSS rules
generated during island SSR via (collect! "cssx" ...). Registered
clear-collected! primitive for the flush component.

Standard CSSX classes now styled server-side. Custom colour shades
(e.g. text-violet-699) still need investigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -184,6 +184,16 @@ let () = Sx_primitives.register "collected" (fun args ->
     (match stack with List items :: _ -> List items | _ -> List [])
   | _ -> List [])
 
+let () = Sx_primitives.register "clear-collected!" (fun args ->
+  match args with
+  | [String name] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with
+     | _ :: rest -> Hashtbl.replace _scope_stacks name (List [] :: rest)
+     | [] -> Hashtbl.replace _scope_stacks name [List []]);
+    Nil
+  | _ -> Nil)
+
 let () = Sx_primitives.register "scope-emit!" (fun args ->
   match args with
   | [String name; value] ->