Rename the app shell to the scaffold and give it a FAB slot

Plan step 35: <x-app-shell> becomes <x-scaffold>, M3's and Compose's name
for the structure of bars, rails and panes, with no alias; every package
view, the showcase, the tests, the skills, README and UPGRADE follow. Its
markup and Tailwind classes stay for the navigation rewrite. The new fab
slot places the FAB as Compose's Scaffold does - bottom-end, 16px from the
edges below medium and 24px from it, clear of the navigation bar and of a
snackbar on screen (resources/css/layout/scaffold.css) - and the content
region tells a pane inside that the margin is already drawn.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/css/components/navigation.css

@@ -44,7 +44,7 @@
  * the same 37.5rem, so a bar in a narrow column lays out by its own width.
  *
  * Everything here is in `@layer components` except the blocks after it, which reach past the
- * component into what an application wrote — the bottom offset <x-app-shell> publishes, a FAB put
+ * component into what an application wrote — the bottom offset <x-scaffold> publishes, a FAB put
  * in the rail: those have to beat a utility, and a layer never does.
  */
 
@@ -296,7 +296,7 @@
     }
 
     /* `--material-banner` is the height of a bar pinned across the top of the window — what
-       <x-app-shell>'s `banner` slot holds, if the application made it sticky. The rail sticks
+       <x-scaffold>'s `banner` slot holds, if the application made it sticky. The rail sticks
        under it rather than behind it; 0 without one. */
     [data-navigation-rail-panel] {
         position: sticky;
@@ -795,7 +795,7 @@
 /* A bar that has slid off the bottom of the window is no longer there to clear, so the offset
  * everything pinned to the bottom reads drops to the bottom safe area and whatever the application
  * has docked on the bar — a `fab` button, the snackbar and the page's own bottom padding all follow
- * it down and come back up with the bar. Unlayered on purpose: <x-app-shell> publishes the variable
+ * it down and come back up with the bar. Unlayered on purpose: <x-scaffold> publishes the variable
  * with a utility, and a rule in any layer loses to one. */
 [data-app-shell]:has([data-navigation-bar][data-hide-on-scroll][data-hidden]) {
     --material-bottom-bar: calc(var(--material-safe-bottom, env(safe-area-inset-bottom)) + var(--material-bottom-extra, 0px));