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>

relations/bp/fragments/routes.py

@@ -25,15 +25,15 @@ def register():
     async def get_fragment(fragment_type: str):
         handler = _handlers.get(fragment_type)
         if handler is None:
-            return Response("", status=200, content_type="text/sexp")
+            return Response("", status=200, content_type="text/sx")
         src = await handler()
-        return Response(src, status=200, content_type="text/sexp")
+        return Response(src, status=200, content_type="text/sx")
 
     # --- generic container-nav fragment ----------------------------------------
 
     async def _container_nav_handler():
-        from shared.sexp.helpers import sexp_call
-        from shared.sexp.relations import relations_from
+        from shared.sx.helpers import sx_call
+        from shared.sx.relations import relations_from
         from shared.services.relationships import get_children
         from shared.infrastructure.urls import events_url, market_url
 
@@ -76,7 +76,7 @@ def register():
                     path = f"/{slug}/"
                 url_fn = _SERVICE_URL.get(defn.to_type)
                 href = url_fn(path) if url_fn else path
-                parts.append(sexp_call("relation-nav",
+                parts.append(sx_call("relation-nav",
                     href=href,
                     name=child.label or "",
                     icon=defn.nav_icon or "",