Fix call-lambda signal mutation, rebuild browser JS, reformat reactive-islands

- sx_browser.ml: use cek_call instead of sx_call in call-lambda to
  avoid eval_expr deep-copying Dict values (breaks signal mutation)
- sx-browser.js: rebuilt with latest transpiler changes
- reactive-islands/index.sx: pretty-printed (no semantic changes)

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

hosts/ocaml/browser/sx_browser.ml

@@ -705,8 +705,10 @@ let () =
   bind "call-lambda" (fun args ->
     match args with
     | [f; a; _] | [f; a] when is_callable f ->
-      let arg_list = match a with List l -> l | Nil -> [] | v -> [v] in
-      Sx_ref.trampoline (Sx_runtime.sx_call f arg_list)
+      (* Use cek_call instead of sx_call to avoid eval_expr copying
+         Dict values (signals). sx_call returns a Thunk resolved via
+         eval_expr which deep-copies dicts, breaking signal mutation. *)
+      Sx_ref.cek_call f a
     | _ -> raise (Eval_error "call-lambda: expected (fn args env?)"));
   bind "cek-call" (fun args ->
     match args with

shared/static/scripts/sx-browser.js

@@ -24,7 +24,7 @@
   // =========================================================================
 
   var NIL = Object.freeze({ _nil: true, toString: function() { return "nil"; } });
-  var SX_VERSION = "2026-03-31T08:20:17Z";
+  var SX_VERSION = "2026-03-31T09:56:06Z";
 
   function isNil(x) { return x === NIL || x === null || x === undefined; }
   function isSxTruthy(x) { return x !== false && !isNil(x); }