Size the loading indicator and publish the rail's value

Two gaps found rewriting the Boost skill for 2.0's plain CSS, each
closed the way M3 describes it.

`<x-loading size="96">`: M3 gives the loading indicator a responsive
size, 48dp by default and flexible from 24 to 240dp ("never exceed that
range"), with the container and the active shape in a fixed ratio. The
size had no prop, so an application wrote a width and height of its own;
`size` now takes a whole number of px in that range, written as
`--md-loading-size`, and the SVG keeps the 48:38 ratio as it scales. A
value outside the range is ignored, as `<x-icon>` and `<x-shape>` ignore
theirs, and an application's own width and height still win.

`--md-navigation-rail-value`: M3 Expressive's rail has two values,
collapsed and expanded (Compose's WideNavigationRailValue), and content
in a rail follows it. Without a hook an application copied the rail's
seven conditions — mode, `data-rail`, `data-rail-auto`, open, and the
window band — out of navigation-rail.css. The rail now publishes the
answer from the same branches that narrow it: `expanded` by default,
`collapsed` wherever it is drawn collapsed, so a style query in the
application's CSS switches at the first paint and at the same moment
as the rail's own items. A rail open over a scrim reads `expanded`, and
outside a rail the property is unset. A JS attribute would have missed
the first paint; a width container query would have lagged the collapse
animation.

Style queries on a custom property need Firefox 151, so the browser
floor moves from Firefox 147 to 151 (README, the CI note, UPGRADE's new
2.1.0 section); Chrome 125 and Safari 18.4 are unchanged.

Browser tests pin both in Chrome, Firefox and Safari: the indicator's
drawn box at 96 and 32px, and the rail's value — with a style query
acting on it — at the first paint for fixed modes, across the window
classes and the menu button for an adaptive rail, and open and closed
for a modal one. NavigationRailTest pins the value in each of the five
collapsed branches.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

resources/views/showcase/sections/buttons.blade.php

@@ -60,8 +60,8 @@
         'Loading indicator' => <<<'BLADE'
             <x-loading />
             <x-loading contained />
-            <x-loading label="Uploading" style="width: 96px; height: 96px; color: var(--md-sys-color-tertiary);" />
-            <x-loading contained style="width: 32px; height: 32px;" />
+            <x-loading label="Uploading" size="96" style="color: var(--md-sys-color-tertiary);" />
+            <x-loading contained size="32" />
             BLADE,
         'Button groups: standard (press a button) and connected' => <<<'BLADE'
             <x-button-group label="Standard" size="md">