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

UPGRADE.md

@@ -101,9 +101,10 @@ over a colour, use the role directly.
   `--material-bottom-bar`; a standard toolbar's icon buttons are `primary`.
 - The centred app bar headline is a grid column; the `search` variant bounds its own width, so
   drop hand-written `max-w-*` wrappers.
-- `<x-app-shell>` is a column with a nested row and gains a `banner` slot; `<x-navigation-rail>`'s
-  `header` slot takes one `<x-fab label icon>` that morphs (replace the two-FAB
-  `rail-collapsed:` swap); new `divider` and `fill` props.
+- `<x-app-shell>` is renamed `<x-scaffold>`, with no alias; it is a column with a nested row
+  and gains `banner` and `fab` slots. `<x-navigation-rail>`'s `header` slot takes one
+  `<x-fab label icon>` that morphs (replace the two-FAB `rail-collapsed:` swap); new `divider`
+  and `fill` props.
 - `<x-icon optical="20">` selects the optical-size-20 cut; the components pass it for their own
   small icons, applications pass it for icons drawn at 20px or less.
 - A text field stops at 40rem wide from `medium:`, as M3 bounds fields on wider windows; a