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>

docker-compose.dev-sx.yml

@@ -14,6 +14,7 @@ services:
       RELOAD: "true"
       SX_USE_REF: "1"
       SX_USE_OCAML: "1"
+      SX_USE_WASM: "1"
       SX_OCAML_BIN: "/app/bin/sx_server"
       SX_BOUNDARY_STRICT: "1"
       SX_DEV: "1"