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/toolbar.css

@@ -124,7 +124,7 @@
 /* Placed over the page: centred above the bottom edge, or centred against the end edge.
  *
  * A toolbar at the bottom shares the screen region with a navigation bar, so it clears the one
- * <x-app-shell> publishes as --material-bottom-bar, exactly as the FAB and the snackbar do —
+ * <x-scaffold> publishes as --material-bottom-bar, exactly as the FAB and the snackbar do —
  * `max()`, not a sum, because that height already swallows the bottom safe area; without a bar
  * the safe area alone applies. A *docked* toolbar and a navigation bar must never be on screen
  * together at all (docs/reference/m3/components-navigation-selection-inputs.md § Toolbars); the