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>

cart/sx/calendar.sx

@@ -0,0 +1,12 @@
+;; Cart calendar entry components
+
+(defcomp ~cart-cal-entry (&key name date-str cost)
+  (li :class "flex items-start justify-between text-sm"
+    (div (div :class "font-medium" name)
+      (div :class "text-xs text-stone-500" date-str))
+    (div :class "ml-4 font-medium" cost)))
+
+(defcomp ~cart-cal-section (&key items)
+  (div :class "mt-6 border-t border-stone-200 pt-4"
+    (h2 :class "text-base font-semibold mb-2" "Calendar bookings")
+    (ul :class "space-y-2" items)))