Add spreads page function for SX URL routing

Without this, /sx/(geography.(spreads)) 404s because spreads isn't
defined as a page function to return the content component.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -60,6 +60,10 @@
         "phase2" '(~reactive-islands/phase2/reactive-islands-phase2-content)
         :else '(~reactive-islands/index/reactive-islands-index-content)))))
 
+(define spreads
+  (fn (content)
+    (if (nil? content) '(~geography/spreads-content) content)))
+
 (define marshes
   (fn (content)
     (if (nil? content) '(~reactive-islands/marshes/reactive-islands-marshes-content) content)))