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/infrastructure/fragments.py

@@ -78,15 +78,15 @@ async def fetch_fragment(
 ) -> str:
     """Fetch a fragment from another app.
 
-    Returns an HTML string or a ``SexpExpr`` (when the provider responds
-    with ``text/sexp``).  When *required* is True (default), raises
+    Returns an HTML string or a ``SxExpr`` (when the provider responds
+    with ``text/sx``).  When *required* is True (default), raises
     ``FragmentError`` on network errors or non-200 responses.
     When *required* is False, returns ``""`` on failure.
 
     Automatically returns ``""`` when called inside a fragment request
     to prevent circular dependencies between apps.
     """
-    from shared.sexp.parser import SexpExpr
+    from shared.sx.parser import SxExpr
 
     if _is_fragment_request():
         return ""
@@ -102,8 +102,8 @@ async def fetch_fragment(
         )
         if resp.status_code == 200:
             ct = resp.headers.get("content-type", "")
-            if "text/sexp" in ct:
-                return SexpExpr(resp.text)
+            if "text/sx" in ct:
+                return SxExpr(resp.text)
             return resp.text
         msg = f"Fragment {app_name}/{fragment_type} returned {resp.status_code}"
         if required: