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/bin/integration_tests.ml

@@ -315,7 +315,7 @@ let make_integration_env () =
       let body_env = { bindings = Hashtbl.create 16; parent = Some e } in
       List.iteri (fun i p ->
         let v = if i < List.length macro_args then List.nth macro_args i else Nil in
-        Hashtbl.replace body_env.bindings p v
+        Hashtbl.replace body_env.bindings (Sx_types.intern p) v
       ) m.m_params;
       Sx_ref.eval_expr m.m_body (Env body_env)
     | _ -> raise (Eval_error "expand-macro: expected (macro args env)"));