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/runner.py

@@ -17,7 +17,7 @@ _running: bool = False
 
 # Each service group runs in its own pytest subprocess with its own PYTHONPATH
 _SERVICE_GROUPS: list[dict] = [
-    {"name": "shared", "dirs": ["shared/tests/", "shared/sexp/tests/"],
+    {"name": "shared", "dirs": ["shared/tests/", "shared/sx/tests/"],
      "pythonpath": None},
     {"name": "blog", "dirs": ["blog/tests/"], "pythonpath": "/app/blog"},
     {"name": "market", "dirs": ["market/tests/"], "pythonpath": "/app/market"},