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>

shared/tests/test_config.py

@@ -74,12 +74,12 @@ class TestFreeze:
                 "blog": "https://blog.rose-ash.com",
                 "market": "https://market.rose-ash.com",
             },
-            "features": ["sexp", "federation"],
+            "features": ["sx", "federation"],
             "limits": {"max_upload": 10485760},
         }
         frozen = _freeze(raw)
         assert frozen["app_urls"]["blog"] == "https://blog.rose-ash.com"
-        assert frozen["features"] == ("sexp", "federation")
+        assert frozen["features"] == ("sx", "federation")
         with pytest.raises(TypeError):
             frozen["app_urls"]["blog"] = "changed"