Step 11: define-foreign FFI + transpiler mutable globals fix

FFI: define-foreign special form in evaluator — registry, param parser,
kwargs parser, binding resolver, type checker, lambda builder, dispatcher.
Generates callable lambdas that route through foreign-dispatch to host-call.
24 tests in test-foreign.sx (registry, parsing, resolution, type checking).

Transpiler: fix mutable global ref emission — ml-emit-define now emits
both X_ref = ref <init> and X_ = <init> for starred globals (was missing
the ref definition entirely, broke retranspilation). Add *provide-batch-depth*,
*provide-batch-queue*, *provide-subscribers* to mutable globals list.

Evaluator: add missing (define *provide-batch-queue* (list)) and
(define *provide-subscribers* (dict)) — were only in hand-edited sx_ref.ml.

Known: 36 bind-tracking + 8 capability test failures on retranspilation
(pre-existing transpiler local-ref shadowing bug, not caused by FFI).

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

hosts/ocaml/transpiler.sx

@@ -285,7 +285,14 @@
 
 (define
   ml-mutable-globals
-  (list "*strict*" "*prim-param-types*" "*last-error-kont*" "*bind-tracking*"))
+  (list
+    "*strict*"
+    "*prim-param-types*"
+    "*last-error-kont*"
+    "*bind-tracking*"
+    "*provide-batch-depth*"
+    "*provide-batch-queue*"
+    "*provide-subscribers*"))
 
 (define
   ml-is-mutable-global?
@@ -1824,7 +1831,16 @@
                       "\n")))))
             (if
               (ml-is-mutable-global? name)
-              (str "let " ml-name " =\n  !" ml-name "ref\n")
+              (str
+                "let "
+                ml-name
+                "ref = ref "
+                (ml-expr val-expr)
+                "\nand "
+                ml-name
+                " =\n  "
+                (ml-expr val-expr)
+                "\n")
               (str "let " ml-name " =\n  " (ml-expr val-expr) "\n"))))))))
 
 (define
@@ -1935,5 +1951,14 @@
                     "\n")))))
           (if
             (ml-is-mutable-global? name)
-            (str "let rec " ml-name " =\n  !" ml-name "ref\n")
+            (str
+              "let rec "
+              ml-name
+              "ref = ref "
+              (ml-expr val-expr)
+              "\nand "
+              ml-name
+              " =\n  "
+              (ml-expr val-expr)
+              "\n")
             (str "let rec " ml-name " =\n  " (ml-expr val-expr) "\n")))))))