Add (use) module declarations

- (use module-name) is a no-op at eval time — purely declarative
- find-use-declarations in tree-tools.sx scans files for use forms
- sx_deps now reports declared modules alongside dependency analysis
- Native 'use' binding in MCP server so files with use don't error

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

hosts/ocaml/bin/mcp_tree.ml

@@ -198,6 +198,8 @@ let setup_env () =
         | _ -> Nil
       ) nodes path
     | _ -> Nil);
+  (* use — module declaration, no-op at eval time, metadata for static analysis *)
+  bind "use" (fun _args -> Nil);
   bind "trim" (fun args -> match args with
     | [String s] -> String (String.trim s) | _ -> String "");
   bind "split" (fun args -> match args with
@@ -1131,6 +1133,13 @@ let rec handle_tool name args =
           ) items
         with _ -> ()
     ) all_sx_files;
+    (* Find use declarations *)
+    let use_decls = call_sx "find-use-declarations" [tree] in
+    let declared_modules = match use_decls with
+      | List items | ListRef { contents = items } ->
+        List.filter_map (fun v -> match v with String s -> Some s | _ -> None) items
+      | _ -> []
+    in
     (* Format output *)
     let lines = List.map (fun sym ->
       if Hashtbl.mem file_defines sym then
@@ -1145,8 +1154,11 @@ let rec handle_tool name args =
       | Some n -> Printf.sprintf "Dependencies of %s in %s" n file
       | None -> Printf.sprintf "Dependencies of %s" file
     in
-    text_result (Printf.sprintf "%s\n%d symbols referenced:\n%s"
-      header (List.length sym_names) (String.concat "\n" lines))
+    let use_str = if declared_modules = [] then "" else
+      Printf.sprintf "\n\nDeclared modules (use):\n  %s" (String.concat ", " declared_modules)
+    in
+    text_result (Printf.sprintf "%s\n%d symbols referenced:\n%s%s"
+      header (List.length sym_names) (String.concat "\n" lines) use_str)
 
   | "sx_build_manifest" ->
     let target = (try args |> member "target" |> to_string with _ -> "js") in