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>

test/sx/components.sx

@@ -22,9 +22,7 @@
 
 ;; Layout: full page header stack (reads root header values from env free variables)
 (defcomp ~test-layout-full (&key services active-service)
-  (<> (~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 (~test-header-row :services services :active-service active-service))))
 
@@ -93,9 +91,7 @@
 
 ;; Detail page header stack (reads root header values from env free variables)
 (defcomp ~test-detail-layout-full (&key services test-nodeid test-label)
-  (<> (~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 (<> (~test-header-row :services services)
                (~header-child-sx :id "test-header-child"