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/__init__.py

@@ -148,6 +148,7 @@ def _register_sx_helpers() -> None:
     from shared.sx.pages import register_page_helpers
     from sxc.sx_components import (
         _docs_content_sx, _reference_content_sx,
+        _reference_index_sx, _reference_attr_detail_sx,
         _protocol_content_sx, _examples_content_sx,
         _essay_content_sx,
         _docs_nav_sx, _reference_nav_sx,
@@ -189,6 +190,8 @@ def _register_sx_helpers() -> None:
         "home-content": _home_content,
         "docs-content": _docs_content_sx,
         "reference-content": _reference_content_sx,
+        "reference-index-content": _reference_index_sx,
+        "reference-attr-detail": _reference_attr_detail_sx,
         "protocol-content": _protocol_content_sx,
         "examples-content": _examples_content_sx,
         "essay-content": _essay_content_sx,