Stepper persistence: use def-store instead of broken cookies

The home stepper's step-idx signal was not persisting across SX
navigation because set-cookie/freeze-to-sx wasn't working in the
WASM kernel. Replace with def-store which uses a global registry
that survives island re-hydration.

Also fix sx_http.exe build: add sx_http back to dune, inline scope
primitives (Sx_scope module was removed), add declarative form
stubs and render stubs, fix /sx/ home route mapping.

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

hosts/ocaml/bin/dune

@@ -1,5 +1,5 @@
 (executables
- (names run_tests debug_set sx_server integration_tests)
+ (names run_tests debug_set sx_server sx_http integration_tests)
  (libraries sx unix))
 
 (executable

hosts/ocaml/bin/sx_http.ml

@@ -74,7 +74,61 @@ let setup_io_stubs env =
 let make_http_env () =
   let env = make_env () in
   Sx_render.setup_render_env env;
-  Sx_scope.setup_scope_env env;
+  (* Scope primitives — inline since Sx_scope was merged *)
+  let _scope_stacks : (string, Sx_types.value list) Hashtbl.t = Hashtbl.create 8 in
+  let bind name fn = ignore (Sx_types.env_bind env name (Sx_types.NativeFn (name, fn))) in
+  bind "scope-push!" (fun args -> match args with
+    | [String name; value] -> let s = try Hashtbl.find _scope_stacks name with Not_found -> [] in Hashtbl.replace _scope_stacks name (value :: s); Nil
+    | [String name] -> let s = try Hashtbl.find _scope_stacks name with Not_found -> [] in Hashtbl.replace _scope_stacks name (Nil :: s); Nil
+    | _ -> Nil);
+  bind "scope-pop!" (fun args -> match args with
+    | [String name] -> (match (try Hashtbl.find _scope_stacks name with Not_found -> []) with _ :: rest -> Hashtbl.replace _scope_stacks name rest | [] -> ()); Nil
+    | _ -> Nil);
+  bind "scope-peek" (fun args -> match args with
+    | [String name] -> (match (try Hashtbl.find _scope_stacks name with Not_found -> []) with v :: _ -> v | [] -> Nil)
+    | _ -> Nil);
+  bind "scope-emit!" (fun args -> match args with
+    | [String name; value] ->
+      let key = name ^ ":emitted" in
+      let s = try Hashtbl.find _scope_stacks key with Not_found -> [] in
+      Hashtbl.replace _scope_stacks key (value :: s); Nil
+    | _ -> Nil);
+  bind "scope-emitted" (fun args -> match args with
+    | [String name] ->
+      let key = name ^ ":emitted" in
+      let items = try Hashtbl.find _scope_stacks key with Not_found -> [] in
+      Hashtbl.replace _scope_stacks key []; List (List.rev items)
+    | _ -> List []);
+  bind "collect!" (fun args -> match args with
+    | [String name; value] ->
+      let key = name ^ ":collected" in
+      let s = try Hashtbl.find _scope_stacks key with Not_found -> [] in
+      Hashtbl.replace _scope_stacks key (value :: s); Nil
+    | _ -> Nil);
+  bind "collected" (fun args -> match args with
+    | [String name] ->
+      let key = name ^ ":collected" in
+      let items = try Hashtbl.find _scope_stacks key with Not_found -> [] in
+      Hashtbl.replace _scope_stacks key []; List (List.rev items)
+    | _ -> List []);
+  (* Declarative form stubs — no-ops at runtime *)
+  bind "define-module" (fun _args -> Nil);
+  bind "define-primitive" (fun _args -> Nil);
+  bind "deftype" (fun _args -> Nil);
+  bind "defeffect" (fun _args -> Nil);
+  bind "deftest" (fun _args -> Nil);
+  bind "defstyle" (fun _args -> Nil);
+  bind "defhandler" (fun _args -> Nil);
+  bind "defpage" (fun _args -> Nil);
+  bind "defquery" (fun _args -> Nil);
+  bind "defaction" (fun _args -> Nil);
+  bind "defrelation" (fun _args -> Nil);
+  (* Render stubs *)
+  bind "set-render-active!" (fun _args -> Nil);
+  bind "render-active?" (fun _args -> Bool true);
+  bind "trampoline" (fun args -> match args with
+    | [Thunk (expr, e)] -> Sx_ref.eval_expr expr (Env e)
+    | [v] -> v | _ -> Nil);
   (* Setup all the standard primitives *)
   (* Evaluator bridge — needed for aser, macro expansion *)
   ignore (env_bind env "eval-expr" (NativeFn ("eval-expr", fun args ->
@@ -236,7 +290,7 @@ let sx_render_to_html expr env =
 let render_page env statics path =
   let t0 = Unix.gettimeofday () in
   (* Build the page AST: evaluate the URL path as an SX expression *)
-  let path_expr = if path = "/" || path = "" then "home"
+  let path_expr = if path = "/" || path = "" || path = "/sx/" || path = "/sx" then "home"
     else begin
       (* /sx/(geography.(reactive)) → (geography (reactive)) *)
       let p = if String.length path > 4 && String.sub path 0 4 = "/sx/" then