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/views/components/navigation-rail.blade.php

@@ -29,7 +29,7 @@
      - `modal` — collapsed in the layout; the menu button (or `$store.rail.show()` from anywhere)
        opens it expanded over a scrim, holding focus until Escape, the scrim, the menu button or
        leaving the page closes it (Compose's ModalWideNavigationRail).
-     - `adaptive` — what `<x-app-shell>` uses, one rail per M3 window size class: on a compact
+     - `adaptive` — what `<x-scaffold>` uses, one rail per M3 window size class: on a compact
        window (below `medium`, 600px) nothing until `$store.rail.show()` slides it in as a modal;
        at `medium` (600–839) collapsed in the layout, opening as a modal; at `expanded` (840–1199)
        a standard rail, collapsed until its menu button expands it in place; from `large` (1200)