Fix transpiler append! emit for mutable globals + run_with_io error recovery

The transpiler's append! emit path didn't check ml-is-mutable-global?,
so (append! *provide-batch-queue* sub) wrote to a dead local variable
instead of the global _ref. This caused the combined test suite hang —
fire-provide-subscribers was silently broken before the local-ref shadow
removal, and now correctly modifies the global batch queue.

Also adds run_with_io error-to-raise conversion (kont_has_handler guard)
so native Eval_errors can be caught by CEK guard/handler-bind when running
through the test runner's IO-aware step loop.

2798/2800 tests pass. 2 foreign-type-checking failures remain: guard can't
catch Eval_error from native fns called through cek_run_iterative (the
handler dispatch itself uses cek_call which re-enters cek_run_iterative,
creating an infinite loop). Fix requires spec-level change: make (error)
use CEK raise instead of host-error.

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

hosts/ocaml/transpiler.sx

@@ -834,30 +834,40 @@
             (ml-emit-map-dict args set-vars)
             (= op "append!")
             (let
-              ((target (nth args 0))
+              ((target (symbol-name (nth expr 1)))
                 (item-expr (ml-expr-inner (nth args 1) set-vars)))
               (if
-                (and
-                  (= (type-of target) "symbol")
-                  (some
-                    (fn (v) (= v (ml-mangle (symbol-name target))))
-                    set-vars))
+                (ml-is-mutable-global? target)
                 (let
-                  ((mangled (ml-mangle (symbol-name target))))
+                  ((mangled (ml-mangle target)))
                   (str
                     "("
                     mangled
-                    " := sx_append_b !"
+                    "ref := sx_append_b !"
                     mangled
-                    " "
+                    "ref "
                     item-expr
                     "; Nil)"))
-                (str
-                  "(sx_append_b "
-                  (ml-expr-inner target set-vars)
-                  " "
-                  item-expr
-                  ")")))
+                (if
+                  (and
+                    (= (type-of (nth expr 1)) "symbol")
+                    (some (fn (v) (= v (ml-mangle target))) set-vars))
+                  (let
+                    ((mangled (ml-mangle target)))
+                    (str
+                      "("
+                      mangled
+                      " := sx_append_b !"
+                      mangled
+                      " "
+                      item-expr
+                      "; Nil)"))
+                  (str
+                    "(sx_append_b "
+                    (ml-expr-inner (nth expr 1) set-vars)
+                    " "
+                    item-expr
+                    ")"))))
             (= op "dict-set!")
             (str
               "(sx_dict_set_b "