Draw the scaffold without Tailwind

Plan step 36 (navigation group, third batch): <x-scaffold>'s own
styling moves into resources/css/layout/scaffold.css (the content
region, the bar and rail row, the banner, the actions row and its
rail-collapsed column layout, --material-bottom-bar and
--material-margin publishing, the skip link) alongside step 35's FAB
and content-margin rules already there. Every data-app-shell* hook
becomes data-md-scaffold-* (data-app-shell-bar, -actions, -banner);
the skip link is data-md-skip-link; data-app-shell itself is dropped,
data-md-scaffold already named the root.

The actions row's rail-collapsed:flex-col is written out branch for
branch as the navigation rail's own rewrite did for its internal
parts: the three width-independent conditions in one :where() group,
the four width-gated ones each in their own @media block. With that
gone, resources/css/tailwind.css's rail-collapsed custom-variant
shim (its last use) is removed; tailwind.css now carries only
tokens/theme.css and tokens/utilities.css, which the showcase still
needs until step 38.

navigation-bar.css's hide-on-scroll rule reading --material-bottom-bar
stayed unlayered only because <x-scaffold> published that variable
through a Tailwind utility, which no layered rule could outrank; now
scaffold.css sets it itself in material.layout, a layer
navigation-bar.css's own material.components always beats, so the
rule moves into the layer and the file fits one
@layer material.components block like every other navigation
stylesheet. navigation-bar rejoins NavigationStylesheetsTest.php's
dataset and NavigationBarTest.php's own duplicate shape test is
retired in favour of it.

Browser tests added (docs/plans/material-3-browser-tests.md): the
scaffold's FAB dropping the bar's own height once hide-bar-on-scroll
slides it away, at the trailing edge in a right-to-left document, and
clearing a safe area an application sets on its inline-end and bottom
edges.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

UPGRADE.md

@@ -106,7 +106,9 @@ over a colour, use the role directly.
 - `<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.
+  and `fill` props. `data-app-shell`, `data-app-shell-bar`, `data-app-shell-actions` and
+  `data-app-shell-banner` are `data-md-scaffold`, `data-md-scaffold-bar`, `data-md-scaffold-actions`
+  and `data-md-scaffold-banner`; the skip link is `data-md-skip-link`.
 - `<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