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>

shared/sx/layouts.py

@@ -104,16 +104,18 @@ def _post_full(ctx: dict, **kw: Any) -> str:
 
 def _post_oob(ctx: dict, **kw: Any) -> str:
     post_hdr = post_header_sx(ctx, oob=True)
-    return post_hdr
+    # Also replace #post-header-child (empty — clears any nested admin rows)
+    child_oob = oob_header_sx("post-header-child", "", "")
+    return "(<> " + post_hdr + " " + child_oob + ")"
 
 
 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 + ")"
+    post_hdr = post_header_sx(ctx, child=admin_hdr)
+    return "(<> " + root_hdr + " " + post_hdr + ")"
 
 
 def _post_admin_oob(ctx: dict, **kw: Any) -> str: