Add SX editor to post edit page, prevent sx_content clearing on save

- Add sx_content to _post_to_edit_dict so edit page receives existing content
- Add SX/Koenig editor tabs, sx-editor mount point, and SxEditor.mount init
- Only pass sx_content to writer_update when form field is present (prevents
  accidental clearing when editing via Koenig-only path)
- Add csrf_exempt to example API POST/DELETE/PUT demo endpoints
- Add defpage infrastructure (pages.py, layouts.py) and sx docs page definitions
- Add defhandler definitions for example API handlers (examples.sx)

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

blog/bp/post/admin/routes.py

@@ -23,6 +23,7 @@ def _post_to_edit_dict(post) -> dict:
     d: dict = {}
     for col in (
         "id", "slug", "title", "html", "plaintext", "lexical", "mobiledoc",
+        "sx_content",
         "feature_image", "feature_image_alt", "feature_image_caption",
         "excerpt", "custom_excerpt", "visibility", "status", "featured",
         "is_page", "email_only", "canonical_url",
@@ -595,6 +596,10 @@ def register():
       effective_status = status
 
     sx_content_raw = form.get("sx_content", "").strip() or None
+    # Build optional kwargs — only pass sx_content if the form field was present
+    extra_kw: dict = {}
+    if "sx_content" in form:
+      extra_kw["sx_content"] = sx_content_raw
     try:
       post = await writer_update(
           g.s,
@@ -606,7 +611,7 @@ def register():
           custom_excerpt=custom_excerpt or None,
           feature_image_caption=feature_image_caption or None,
           status=effective_status,
-          sx_content=sx_content_raw,
+          **extra_kw,
       )
     except OptimisticLockError:
       return redirect(