Fix SX history, OOB header swaps, cross-service nav components

- Always re-fetch on popstate (drop LRU cache) for fresh content on back/forward
- Save/restore scroll position via pushState
- Add id="root-header-child" to ~app-body so OOB swaps can target it
- Fix OOB renderers: nest root-row inside root-header-child swap instead of
  separate OOB that clobbers it
- Fix 3+ header rows dropped: wrap all headers in single fragment instead of
  concatenating outside (<> ...)
- Strip <script data-components> from text/sx responses before renderToString
- Fall back to location.assign for cross-origin pushState (SecurityError)
- Move blog/sx/nav.sx to shared/sx/templates/ so all services have nav components

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

shared/sx/helpers.py

@@ -209,9 +209,13 @@ def post_admin_header_sx(ctx: dict, slug: str, *, oob: bool = False,
 
 
 def oob_header_sx(parent_id: str, child_id: str, row_sx: str) -> str:
-    """Wrap a header row sx in an OOB swap."""
+    """Wrap a header row sx in an OOB swap.
+
+    child_id is accepted for call-site compatibility but no longer used —
+    the child placeholder is created by ~menu-row-sx itself.
+    """
     return sx_call("oob-header-sx",
-        parent_id=parent_id, child_id=child_id,
+        parent_id=parent_id,
         row=SxExpr(row_sx),
     )