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/css/components/loading.css

@@ -4,7 +4,9 @@
  *
  * The drawing is resources/svg/loading-indicator/, ported from androidx Compose Material 3's
  * LoadingIndicator in bin/loading-indicator.mjs (Apache-2.0): LoadingIndicatorTokens' 38px
- * indicator in a 48px container, which is the size here unless the caller's CSS sizes it. It is
+ * indicator in a 48px container, which is the size here unless `size` (`--md-loading-size`, 24 to
+ * 240px, M3's responsive range) or the caller's CSS sizes it; the SVG scales with its box, so the
+ * container and the shape keep their ratio at every size. It is
  * drawn in the text colour, `primary` (ActiveIndicatorColor) unless the caller colours it;
  * `contained` puts it on a `primary-container` circle in `on-primary-container`
  * (ContainedContainerColor, ContainedIndicatorColor), for a spinner over content.
@@ -21,8 +23,8 @@
         flex-shrink: 0;
         align-items: center;
         justify-content: center;
-        inline-size: 48px;
-        block-size: 48px;
+        inline-size: var(--md-loading-size, 48px);
+        block-size: var(--md-loading-size, 48px);
         color: var(--md-sys-color-primary);
 
         & > svg {