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>

sx/sxc/pages/docs.sx

@@ -0,0 +1,98 @@
+;; SX docs app — declarative page definitions
+;; These replace the GET route handlers in routes.py
+
+;; ---------------------------------------------------------------------------
+;; Home page
+;; ---------------------------------------------------------------------------
+
+(defpage home
+  :path "/"
+  :auth :public
+  :layout :sx
+  :content (home-content))
+
+;; ---------------------------------------------------------------------------
+;; Docs section
+;; ---------------------------------------------------------------------------
+
+(defpage docs-page
+  :path "/docs/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Docs"
+    :sub-label "Docs"
+    :sub-href "/docs/introduction"
+    :sub-nav (docs-nav (find-current DOCS_NAV slug))
+    :selected (or (find-current DOCS_NAV slug) ""))
+  :content (docs-content slug))
+
+;; ---------------------------------------------------------------------------
+;; Reference section
+;; ---------------------------------------------------------------------------
+
+(defpage reference-index
+  :path "/reference/"
+  :auth :public
+  :layout (:sx-section
+    :section "Reference"
+    :sub-label "Reference"
+    :sub-href "/reference/"
+    :sub-nav (reference-nav "Attributes")
+    :selected "Attributes")
+  :content (reference-content ""))
+
+(defpage reference-page
+  :path "/reference/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Reference"
+    :sub-label "Reference"
+    :sub-href "/reference/"
+    :sub-nav (reference-nav (find-current REFERENCE_NAV slug))
+    :selected (or (find-current REFERENCE_NAV slug) ""))
+  :content (reference-content slug))
+
+;; ---------------------------------------------------------------------------
+;; Protocols section
+;; ---------------------------------------------------------------------------
+
+(defpage protocol-page
+  :path "/protocols/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Protocols"
+    :sub-label "Protocols"
+    :sub-href "/protocols/wire-format"
+    :sub-nav (protocols-nav (find-current PROTOCOLS_NAV slug))
+    :selected (or (find-current PROTOCOLS_NAV slug) ""))
+  :content (protocol-content slug))
+
+;; ---------------------------------------------------------------------------
+;; Examples section
+;; ---------------------------------------------------------------------------
+
+(defpage examples-page
+  :path "/examples/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Examples"
+    :sub-label "Examples"
+    :sub-href "/examples/click-to-load"
+    :sub-nav (examples-nav (find-current EXAMPLES_NAV slug))
+    :selected (or (find-current EXAMPLES_NAV slug) ""))
+  :content (examples-content slug))
+
+;; ---------------------------------------------------------------------------
+;; Essays section
+;; ---------------------------------------------------------------------------
+
+(defpage essay-page
+  :path "/essays/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Essays"
+    :sub-label "Essays"
+    :sub-href "/essays/sx-sucks"
+    :sub-nav (essays-nav (find-current ESSAYS_NAV slug))
+    :selected (or (find-current ESSAYS_NAV slug) ""))
+  :content (essay-content slug))