- 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>