Fix WASM reactive signals: unify context/scope, fix flush-subscribers

Three root causes for reactive attribute updates not propagating in WASM:

1. `context` CEK special form only searched kont provide frames, missing
   `scope-push!` entries in the native scope_stacks hashtable. Unified by
   adding scope_stacks fallback to step_sf_context.

2. `flush-subscribers` used bare `(sub)` call which failed to invoke
   complex closures in for-each HO callbacks. Changed to `(cek-call sub nil)`.

3. Test eagerly evaluated `(deref s)` before render-to-dom saw it.
   Fixed tests to use quoted expressions matching real browser boot.

WASM native: 10/10, WASM shell: 26/26.

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

hosts/ocaml/lib/sx_scope.ml

@@ -77,16 +77,24 @@ let () =
 
   register "context" (fun args ->
     match args with
-    | [String name] | [String name; _] ->
+    | (String name) :: rest ->
       let stack = try Hashtbl.find scope_stacks name with Not_found -> [] in
       if !_scope_trace then
         _scope_log := Printf.sprintf "CTX %s depth=%d found=%b" name (List.length stack) (stack <> []) :: !_scope_log;
-      (match stack, args with
-       | v :: _, _ -> v
-       | [], [_; default_val] -> default_val
-       | [], _ -> Nil)
+      (match stack with
+       | v :: _ -> v
+       | [] -> (match rest with default_val :: _ -> default_val | [] -> Nil))
     | _ -> Nil);
 
+  register "context-debug" (fun args ->
+    match args with
+    | [String name] ->
+      let stack = try Hashtbl.find scope_stacks name with Not_found -> [] in
+      let all_keys = Hashtbl.fold (fun k _ acc -> k :: acc) scope_stacks [] in
+      String (Printf.sprintf "name=%s stack_len=%d all_keys=[%s]"
+        name (List.length stack) (String.concat "," all_keys))
+    | _ -> String "bad args");
+
   (* --- Collect / collected / clear-collected! --- *)
 
   register "collect!" (fun args ->