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>

shared/sx/layouts.py

@@ -0,0 +1,139 @@
+"""
+Named layout presets for defpage.
+
+Each layout generates header rows for full-page and OOB rendering.
+Layouts wrap existing helper functions from ``shared.sx.helpers`` so
+defpage can reference them by name (e.g. ``:layout :root``).
+
+Layouts are registered in ``_LAYOUT_REGISTRY`` and looked up by
+``get_layout()`` at request time.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Callable, Awaitable
+
+from .helpers import (
+    root_header_sx, post_header_sx, post_admin_header_sx,
+    oob_header_sx, header_child_sx,
+)
+
+
+# ---------------------------------------------------------------------------
+# Layout protocol
+# ---------------------------------------------------------------------------
+
+class Layout:
+    """A named layout that generates header rows for full and OOB rendering."""
+
+    __slots__ = ("name", "_full_fn", "_oob_fn")
+
+    def __init__(
+        self,
+        name: str,
+        full_fn: Callable[..., str | Awaitable[str]],
+        oob_fn: Callable[..., str | Awaitable[str]],
+    ):
+        self.name = name
+        self._full_fn = full_fn
+        self._oob_fn = oob_fn
+
+    async def full_headers(self, ctx: dict, **kwargs: Any) -> str:
+        result = self._full_fn(ctx, **kwargs)
+        if hasattr(result, "__await__"):
+            result = await result
+        return result
+
+    async def oob_headers(self, ctx: dict, **kwargs: Any) -> str:
+        result = self._oob_fn(ctx, **kwargs)
+        if hasattr(result, "__await__"):
+            result = await result
+        return result
+
+    def __repr__(self) -> str:
+        return f"<Layout:{self.name}>"
+
+
+# ---------------------------------------------------------------------------
+# Registry
+# ---------------------------------------------------------------------------
+
+_LAYOUT_REGISTRY: dict[str, Layout] = {}
+
+
+def register_layout(layout: Layout) -> None:
+    """Register a layout preset."""
+    _LAYOUT_REGISTRY[layout.name] = layout
+
+
+def get_layout(name: str) -> Layout | None:
+    """Look up a layout by name."""
+    return _LAYOUT_REGISTRY.get(name)
+
+
+# ---------------------------------------------------------------------------
+# Built-in layouts
+# ---------------------------------------------------------------------------
+
+def _root_full(ctx: dict, **kw: Any) -> str:
+    return root_header_sx(ctx)
+
+
+def _root_oob(ctx: dict, **kw: Any) -> str:
+    root_hdr = root_header_sx(ctx)
+    return oob_header_sx("root-header-child", "root-header-child", root_hdr)
+
+
+def _post_full(ctx: dict, **kw: Any) -> str:
+    root_hdr = root_header_sx(ctx)
+    post_hdr = post_header_sx(ctx)
+    return "(<> " + root_hdr + " " + post_hdr + ")"
+
+
+def _post_oob(ctx: dict, **kw: Any) -> str:
+    post_hdr = post_header_sx(ctx, oob=True)
+    return post_hdr
+
+
+def _post_admin_full(ctx: dict, **kw: Any) -> str:
+    slug = ctx.get("post", {}).get("slug", "")
+    selected = kw.get("selected", "")
+    root_hdr = root_header_sx(ctx)
+    post_hdr = post_header_sx(ctx)
+    admin_hdr = post_admin_header_sx(ctx, slug, selected=selected)
+    return "(<> " + root_hdr + " " + post_hdr + " " + admin_hdr + ")"
+
+
+def _post_admin_oob(ctx: dict, **kw: Any) -> str:
+    slug = ctx.get("post", {}).get("slug", "")
+    selected = kw.get("selected", "")
+    post_hdr = post_header_sx(ctx, oob=True)
+    admin_hdr = post_admin_header_sx(ctx, slug, selected=selected)
+    admin_oob = oob_header_sx("post-header-child", "post-admin-header-child", admin_hdr)
+    return "(<> " + post_hdr + " " + admin_oob + ")"
+
+
+register_layout(Layout("root", _root_full, _root_oob))
+register_layout(Layout("post", _post_full, _post_oob))
+register_layout(Layout("post-admin", _post_admin_full, _post_admin_oob))
+
+
+# ---------------------------------------------------------------------------
+# Callable layout — services register custom Python layout functions
+# ---------------------------------------------------------------------------
+
+_CUSTOM_LAYOUTS: dict[str, tuple] = {}  # name → (full_fn, oob_fn)
+
+
+def register_custom_layout(name: str,
+                           full_fn: Callable[..., str | Awaitable[str]],
+                           oob_fn: Callable[..., str | Awaitable[str]]) -> None:
+    """Register a custom layout function.
+
+    Used by services with non-standard header patterns::
+
+        register_custom_layout("sx-section",
+            full_fn=my_full_headers,
+            oob_fn=my_oob_headers)
+    """
+    register_layout(Layout(name, full_fn, oob_fn))