Fix duplicate headers on HTMX nav, editor content loading, and double mount

- Nest admin header inside post-header-child (layouts.py/helpers.py) so
  full-page DOM matches OOB swap structure, eliminating duplicate headers
- Clear post-header-child on post layout OOB to remove stale admin rows
- Read SX initial content from #sx-content-input instead of
  window.__SX_INITIAL__ to avoid escaping issues through SX pipeline
- Fix client-side SX parser RE_STRING to handle escaped newlines
- Clear root element in SxEditor.mount() to prevent double content on
  HTMX re-mount
- Remove unused ~blog-editor-sx-initial component

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

blog/sx/editor.sx

@@ -153,10 +153,6 @@
     "  sync();"
     "})();"))
 
-;; SX initial content script for edit form
-(defcomp ~blog-editor-sx-initial (&key sx-content)
-  (script (str "window.__SX_INITIAL__ = '" sx-content "' || null;")))
-
 (defcomp ~blog-editor-styles (&key css-href)
   (<> (link :rel "stylesheet" :href css-href)
     (style