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>

market/sx/meta.sx

@@ -0,0 +1,19 @@
+;; Market meta/SEO components
+
+(defcomp ~market-meta-title (&key title)
+  (title title))
+
+(defcomp ~market-meta-description (&key description)
+  (meta :name "description" :content description))
+
+(defcomp ~market-meta-canonical (&key href)
+  (link :rel "canonical" :href href))
+
+(defcomp ~market-meta-og (&key property content)
+  (meta :property property :content content))
+
+(defcomp ~market-meta-twitter (&key name content)
+  (meta :name name :content content))
+
+(defcomp ~market-meta-jsonld (&key json)
+  (script :type "application/ld+json" (~rich-text :html json)))