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>

sx/Dockerfile

@@ -43,6 +43,11 @@ RUN cp -r sx-app-tmp/app.py sx-app-tmp/path_setup.py \
     ([ -d sx-app-tmp/sx ] && cp -r sx-app-tmp/sx ./sx || true) && \
     rm -rf sx-app-tmp
 
+# SX spec, library, and web adapter files (loaded by OCaml kernel)
+COPY spec/ ./spec/
+COPY lib/ ./lib/
+COPY web/ ./web/
+
 # Sibling models for cross-domain SQLAlchemy imports
 COPY blog/__init__.py     ./blog/__init__.py
 COPY blog/models/         ./blog/models/