Fix navigation: deep URL routing, back button, render timeout

- request-handler.sx: replace all dots (not just `.(`) and auto-quote
  undefined symbols as strings so 3-level URLs like
  /sx/(geography.(reactive.(examples.counter))) resolve correctly
- sx-platform.js: register popstate handler (was missing from manual
  boot sequence) and fetch full HTML for back/forward navigation
- sx_ref.ml: add CEK step limit (10M steps) checked every 4096 steps
  so runaway renders return 500 instead of blocking the worker forever
- Rename test-runner.sx → runner-placeholder.sx to avoid `test-` skip
- Playwright config: pin testDir, single worker, ignore worktrees

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

hosts/ocaml/lib/sx_ref.ml

@@ -30,6 +30,12 @@ let _last_error_kont : value ref = ref Nil
 
 (* === Transpiled from evaluator (frames + eval + CEK) === *)
 
+(* Per-domain step limit (0 = no limit).
+   Set by the HTTP render worker before each page render.
+   Checked every 4096 CEK steps in cek_run. *)
+let _step_limit : int Atomic.t = Atomic.make 0
+let _step_count : int Atomic.t = Atomic.make 0
+
 (* make-cek-state *)
 let rec make_cek_state control env kont =
   (CekState { cs_control = control; cs_env = env; cs_kont = kont; cs_phase = "eval"; cs_value = Nil })
@@ -372,13 +378,23 @@ and sf_provide args env =
 and expand_macro mac raw_args env =
   (let local = (env_merge ((macro_closure (mac))) (env)) in (let () = ignore ((List.iter (fun pair -> ignore ((env_bind local (sx_to_string (first (pair))) (if sx_truthy ((prim_call "<" [(nth (pair) ((Number 1.0))); (len (raw_args))])) then (nth (raw_args) ((nth (pair) ((Number 1.0))))) else Nil)))) (sx_to_list (List (List.mapi (fun i p -> let i = Number (float_of_int i) in (List [p; i])) (sx_to_list (macro_params (mac)))))); Nil)) in (let () = ignore ((if sx_truthy ((macro_rest_param (mac))) then (env_bind local (sx_to_string (macro_rest_param (mac))) (prim_call "slice" [raw_args; (len ((macro_params (mac))))])) else Nil)) in (trampoline ((eval_expr ((macro_body (mac))) (local)))))))
 
-(* cek-run *)
+(* cek-run — iterative loop to avoid JS stack overflow in WASM *)
 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))
+  let s = ref state in
+  (try
+    while not (match cek_terminal_p !s with Bool true -> true | _ -> false) do
+      s := cek_step !s;
+      let n = Atomic.fetch_and_add _step_count 1 in
+      if n land 4095 = 0 then begin
+        let lim = Atomic.get _step_limit in
+        if lim > 0 && n >= lim then
+          raise (Eval_error (Printf.sprintf "Render step limit exceeded (%d steps)" n))
+      end
+    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 =