Transpiler local-ref shadowing fix + foreign test runner bindings

ml-scan-set now checks ml-is-mutable-global? before adding set!/append!
targets to the needs-ref list. Previously, mutable globals like
*bind-tracking* got local `ref Nil` shadows that masked the global _ref,
causing `append!: expected list, got nil` in 43 bind-tracking tests.

Test runner: bind foreign registry functions (foreign-registered?,
foreign-lookup, foreign-names, foreign-register!, foreign-resolve-binding,
foreign-check-args, foreign-build-lambda) + initialize _cek_call_ref for
with-capabilities. 22/24 foreign tests now pass, 8 capabilities tests fixed.

Retranspiled sx_ref.ml — all mutable global shadows eliminated.

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

hosts/ocaml/transpiler.sx

@@ -417,11 +417,13 @@
             (= (symbol-name head) "set!")
             (>= (len node) 2))
           (let
-            ((var-name (if (= (type-of (nth node 1)) "symbol") (symbol-name (nth node 1)) (str (nth node 1)))))
+            ((var-name (symbol-name (nth node 1))))
             (let
               ((mangled (ml-mangle var-name)))
               (when
-                (not (some (fn (x) (= x mangled)) result))
+                (and
+                  (not (ml-is-mutable-global? var-name))
+                  (not (some (fn (x) (= x mangled)) result)))
                 (append! result mangled))))
           (and
             (= (type-of head) "symbol")
@@ -433,7 +435,9 @@
             (let
               ((mangled (ml-mangle var-name)))
               (when
-                (not (some (fn (x) (= x mangled)) result))
+                (and
+                  (not (ml-is-mutable-global? var-name))
+                  (not (some (fn (x) (= x mangled)) result)))
                 (append! result mangled))))
           :else (for-each
             (fn (child) (when (list? child) (ml-scan-set child result)))