Rebrand sexp → sx across web platform (173 files)

Rename all sexp directories, files, identifiers, and references to sx.
artdag/ excluded (separate media processing DSL).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/Dockerfile

@@ -26,7 +26,7 @@ COPY shared/ ./shared/
 COPY test/ ./test-app-tmp/
 # Move service files into /app (flatten), but keep Dockerfile.* in place
 RUN cp -r test-app-tmp/app.py test-app-tmp/path_setup.py \
-         test-app-tmp/bp test-app-tmp/sexp test-app-tmp/services \
+         test-app-tmp/bp test-app-tmp/sx test-app-tmp/services \
          test-app-tmp/runner.py test-app-tmp/__init__.py ./ 2>/dev/null || true && \
     rm -rf test-app-tmp