Fix WASM stack overflow: make cek_run iterative

The CEK evaluator's cek_run was recursive (calls itself via cek_step).
Native OCaml handles deep recursion but wasm_of_ocaml compiles to JS
which has ~10K frame stack limit. Complex expressions (bytecode compiler)
exceeded this, causing "Maximum call stack size exceeded" in all WASM
bytecode compilation.

Replace recursive cek_run with iterative while loop — same semantics,
zero stack growth. Fixes sx_build_bytecode and browser-side evaluation.

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

hosts/ocaml/lib/sx_ref.ml

@@ -374,11 +374,16 @@ and expand_macro mac raw_args env =
 
 (* cek-run *)
 and cek_run state =
-  (if sx_truthy ((cek_terminal_p (state))) then (cek_value (state)) else
-   try cek_run ((cek_step (state)))
-   with Eval_error msg ->
-     (if !_last_error_kont = Nil then _last_error_kont := cek_kont state);
-     raise (Eval_error msg))
+  (* Iterative loop — no stack growth. Essential for WASM where JS stack is limited. *)
+  let s = ref state in
+  (try
+    while not (match cek_terminal_p !s with Bool true -> true | _ -> false) do
+      s := cek_step !s
+    done;
+    cek_value !s
+  with Eval_error msg ->
+    (if !_last_error_kont = Nil then _last_error_kont := cek_kont !s);
+    raise (Eval_error msg))
 
 (* cek-step *)
 and cek_step state =