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>

market/sx/layouts.sx

@@ -15,7 +15,13 @@
           :title (get __mctx "market-title")
           :top-slug (get __mctx "top-slug")
           :sub-div (get __mctx "sub-slug"))
-        :nav (get __mctx "desktop-nav")
+        :nav (~market-desktop-nav-from-data
+          :categories (get __mctx "categories")
+          :hx-select (get __mctx "hx-select")
+          :select-colours (get __mctx "select-colours")
+          :all-href (get __mctx "all-href")
+          :all-active (get __mctx "all-active")
+          :admin-href (get __mctx "admin-href"))
         :child-id "market-header-child"
         :oob (unquote oob)))))