WASM browser build: interned env keys, async kernel boot, bundled .sx platform

- Symbol interning in sx_types.ml: env lookups use int keys (intern/unintern)
  to avoid repeated string hashing in scope chain walks
- sx-platform.js: poll for SxKernel availability (WASM init is async)
- shell.sx: load sx_browser.bc.wasm.js when SX_USE_WASM=1
- bundle.sh: fix .sx file paths (web-signals.sx rename)
- browser/dune: target byte+js+wasm modes
- Bundle 23 .sx platform files for browser (dom, signals, router, boot, etc.)

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

hosts/ocaml/lib/sx_vm.ml

@@ -242,8 +242,9 @@ and run vm =
           let name = match consts.(idx) with String s -> s | _ -> "" in
           let v = try Hashtbl.find vm.globals name with Not_found ->
             (* Walk the closure env chain for inner functions *)
+            let id = Sx_types.intern name in
             let rec env_lookup e =
-              try Hashtbl.find e.bindings name
+              try Hashtbl.find e.bindings id
               with Not_found ->
                 match e.parent with Some p -> env_lookup p | None ->
                   try Sx_primitives.get_primitive name
@@ -262,9 +263,10 @@ and run vm =
           (* Write to closure env if the name exists there (mutable closure vars) *)
           let written = match frame.closure.vm_closure_env with
             | Some env ->
+              let id = Sx_types.intern name in
               let rec find_env e =
-                if Hashtbl.mem e.bindings name then
-                  (Hashtbl.replace e.bindings name (peek vm); true)
+                if Hashtbl.mem e.bindings id then
+                  (Hashtbl.replace e.bindings id (peek vm); true)
                 else match e.parent with Some p -> find_env p | None -> false
               in find_env env
             | None -> false
@@ -556,7 +558,7 @@ let jit_compile_lambda (l : lambda) globals =
            Use a shallow copy so we don't pollute the real globals. *)
         let merged = Hashtbl.copy globals in
         let rec inject env =
-          Hashtbl.iter (fun k v -> Hashtbl.replace merged k v) env.bindings;
+          Hashtbl.iter (fun id v -> Hashtbl.replace merged (Sx_types.unintern id) v) env.bindings;
           match env.parent with Some p -> inject p | None -> ()
         in
         inject closure;