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>

lib/tree-tools.sx

@@ -1315,3 +1315,18 @@
                       (for-each (fn (child) (walk child bound)) args)))))))))
       (walk node (dict))
       result)))
+
+(define find-use-declarations :effects ()
+  (fn (nodes)
+    (let ((uses (list)))
+      (for-each (fn (node)
+        (when (and (list? node) (>= (len node) 2)
+                   (= (type-of (first node)) "symbol")
+                   (= (symbol-name (first node)) "use"))
+          (for-each (fn (arg)
+            (cond
+              (= (type-of arg) "symbol") (append! uses (symbol-name arg))
+              (= (type-of arg) "string") (append! uses arg)))
+            (rest node))))
+        (if (list? nodes) nodes (list nodes)))
+      uses)))