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.unit

@@ -8,7 +8,7 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 
 WORKDIR /app
 
-# Node.js for sexp.js parity tests
+# Node.js for sx.js parity tests
 RUN apt-get update && apt-get install -y --no-install-recommends nodejs && \
     rm -rf /var/lib/apt/lists/*