Add continuation specs: delimited (shift/reset) and full (call/cc)

Optional bolt-on extensions to the SX spec. continuations.sx defines
delimited continuations for all targets. callcc.sx defines full call/cc
for targets where it's native (Scheme, Haskell). Shared continuation
type if both are loaded. Wired into specs section of sx-docs.

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

sx/sxc/pages/docs.sx

@@ -331,6 +331,14 @@
               :filename (get item "filename") :href (str "/specs/" (get item "slug"))
               :source (read-spec-file (get item "filename"))))
         browser-spec-items))
+    "extensions" (~spec-overview-content
+      :spec-title "Extensions"
+      :spec-files (map (fn (item)
+        (dict :title (get item "title") :desc (get item "desc")
+              :prose (get item "prose")
+              :filename (get item "filename") :href (str "/specs/" (get item "slug"))
+              :source (read-spec-file (get item "filename"))))
+        extension-spec-items))
     :else (let ((spec (find-spec slug)))
       (if spec
         (~spec-detail-content