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/docs.sx

@@ -35,9 +35,15 @@
             (map (fn (cell) (td :class "px-3 py-2 text-stone-700" cell)) row)))
           rows)))))
 
-(defcomp ~doc-attr-row (&key attr description exists)
+(defcomp ~doc-attr-row (&key attr description exists href)
   (tr :class "border-b border-stone-100"
-    (td :class "px-3 py-2 font-mono text-sm text-violet-700 whitespace-nowrap" attr)
+    (td :class "px-3 py-2 font-mono text-sm whitespace-nowrap"
+      (if href
+        (a :href href
+           :sx-get href :sx-target "#main-panel" :sx-select "#main-panel"
+           :sx-swap "outerHTML" :sx-push-url "true"
+           :class "text-violet-700 hover:text-violet-900 underline" attr)
+        (span :class "text-violet-700" attr)))
     (td :class "px-3 py-2 text-stone-700 text-sm" description)
     (td :class "px-3 py-2 text-center"
       (if exists