WIP: pre-existing changes from WASM browser work + test infrastructure

Accumulated changes from WASM browser development sessions:
- sx_runtime.ml: signal subscription + notify, env unwrap tolerance
- sx_browser.bc.js: rebuilt js_of_ocaml browser kernel
- sx_browser.bc.wasm.js + assets: WASM browser kernel build
- sx-platform.js browser tests (test_js, test_platform, test_wasm)
- Playwright sx-inspect.js: interactive page inspector tool
- harness-web.sx: DOM assertion updates
- deploy.sh, Dockerfile, dune-project: build config updates
- test-stepper.sx: stepper unit tests
- reader-macro-demo plan update

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

shared/static/wasm/sx_browser.bc.wasm.assets/std_exit-10fb8830.wasm.map

@@ -0,0 +1 @@
+{"version":3,"sources":["/builtin/blackbox.ml","/root/.opam/5.2.0/lib/ocaml/std_exit.ml"],"sourcesContent":["(* generated code *)","(**************************************************************************)\n(*                                                                        *)\n(*                                 OCaml                                  *)\n(*                                                                        *)\n(*             Xavier Leroy, projet Cristal, INRIA Rocquencourt           *)\n(*                                                                        *)\n(*   Copyright 1996 Institut National de Recherche en Informatique et     *)\n(*     en Automatique.                                                    *)\n(*                                                                        *)\n(*   All rights reserved.  This file is distributed under the terms of    *)\n(*   the GNU Lesser General Public License version 2.1, with the          *)\n(*   special exception on linking described in the file LICENSE.          *)\n(*                                                                        *)\n(**************************************************************************)\n\n(* Ensure that [at_exit] functions are called at the end of every program *)\n\nlet _ = do_at_exit()\n"],"names":[],"mappings":"6GCiBQ,IDjBR,QCiBQ,cAAY","ignoreList":[0]}