Add attribute detail pages with live demos for SX reference

Per-attribute documentation pages at /reference/attributes/<slug> with:
- Live interactive demos (demo components in reference.sx)
- S-expression source code display
- Server handler code shown as s-expressions (defhandlers in handlers/reference.sx)
- Wire response display via OOB swaps on demo interaction
- Linked attribute names in the reference table

Covers all 20 implemented attributes (sx-get/post/put/delete/patch,
sx-trigger/target/swap/swap-oob/select/confirm/push-url/sync/encoding/
headers/include/vals/media/disable/on:*, sx-retry, data-sx, data-sx-env).

Also adds sx-on:* to BEHAVIOR_ATTRS, updates REFERENCE_NAV to link
/reference/attributes, and makes /reference/ an index page.

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

sx/sxc/pages/docs.sx

@@ -48,9 +48,9 @@
     :section "Reference"
     :sub-label "Reference"
     :sub-href "/reference/"
-    :sub-nav (reference-nav "Attributes")
-    :selected "Attributes")
-  :content (reference-content ""))
+    :sub-nav (reference-nav "")
+    :selected "")
+  :content (reference-index-content))
 
 (defpage reference-page
   :path "/reference/<slug>"
@@ -63,6 +63,17 @@
     :selected (or (find-current REFERENCE_NAV slug) ""))
   :content (reference-content slug))
 
+(defpage reference-attr-detail
+  :path "/reference/attributes/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Reference"
+    :sub-label "Reference"
+    :sub-href "/reference/"
+    :sub-nav (reference-nav "Attributes")
+    :selected "Attributes")
+  :content (reference-attr-detail slug))
+
 ;; ---------------------------------------------------------------------------
 ;; Protocols section
 ;; ---------------------------------------------------------------------------