Fix market header ImportError and sx docs menu bar 3 OOB insertion

- market/sx/layouts.sx: Update ~market-header-auto macro to build nav
  from data fields via ~market-desktop-nav-from-data instead of
  expecting pre-built "desktop-nav" SxExpr (removed in Phase 9)
- shared/sx/primitives_io.py: Import _market_header_data instead of
  deleted _desktop_category_nav_sx, return individual data fields
- sx/sx/layouts.sx: Fix ~sx-section-layout-oob to use ~oob-header-sx
  for inserting sub-row into always-existing container div

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

sx/sx/layouts.sx

@@ -73,8 +73,9 @@
                           :sub-nav sub-nav :selected selected))))
 
 (defcomp ~sx-section-layout-oob (&key section sub-label sub-href sub-nav selected)
-  (<> (~sx-sub-row :sub-label sub-label :sub-href sub-href
-                   :sub-nav sub-nav :selected selected :oob true)
+  (<> (~oob-header-sx :parent-id "sx-header-child"
+        :row (~sx-sub-row :sub-label sub-label :sub-href sub-href
+                          :sub-nav sub-nav :selected selected))
     (~sx-header-row
       :nav (~sx-main-nav :section section)
       :oob true)