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>

sx/sx/layouts.sx

@@ -1,12 +1,10 @@
-;; SX docs layout defcomps — root header from env free variables,
+;; SX docs layout defcomps — root header via ~root-header-auto,
 ;; sx-specific headers passed as &key params.
 
 ;; --- SX home layout: root + sx menu row ---
 
 (defcomp ~sx-layout-full (&key sx-row)
-  (<> (~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)
     sx-row))
 
 (defcomp ~sx-layout-oob (&key root-header sx-row)
@@ -15,7 +13,5 @@
 ;; --- SX section layout: root + sx row (with child sub-row) ---
 
 (defcomp ~sx-section-layout-full (&key sx-row)
-  (<> (~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)
     sx-row))