Replace env free-variable threading with IO-primitive auto-fetch macros

Layout components now self-resolve context (cart-mini, auth-menu, nav-tree,
rights, URLs) via new IO primitives (root-header-ctx, select-colours,
account-nav-ctx, app-rights) and defmacro wrappers (~root-header-auto,
~auth-header-row-auto, ~root-mobile-auto). This eliminates _ctx_to_env(),
HELPER_CSS_CLASSES, and verbose :key threading across all 10 services.

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

federation/sx/layouts.sx

@@ -3,9 +3,7 @@
 
 ;; Full page: root header + social header in header-child
 (defcomp ~social-layout-full ()
-  (<> (~root-header :cart-mini cart-mini :blog-url blog-url :site-title site-title
-                     :app-label app-label :nav-tree nav-tree :auth-menu auth-menu
-                     :nav-panel nav-panel :settings-url settings-url :is-admin is-admin)
+  (<> (~root-header-auto)
     (~header-child-sx
       :inner (~federation-social-header
                :nav (~federation-social-nav :actor actor)))))
@@ -16,7 +14,4 @@
         :parent-id "root-header-child"
         :row (~federation-social-header
                :nav (~federation-social-nav :actor actor)))
-    (~root-header :cart-mini cart-mini :blog-url blog-url :site-title site-title
-                   :app-label app-label :nav-tree nav-tree :auth-menu auth-menu
-                   :nav-panel nav-panel :settings-url settings-url :is-admin is-admin
-                   :oob true)))
+    (~root-header-auto true)))