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>

UPGRADE.md

@@ -1,5 +1,17 @@
 # Upgrading
 
+## From 2.0.0 to 2.1.0
+
+- **Browsers:** Firefox 151 or later (was 147), for container style queries on a custom property;
+  Chrome 125 and Safari 18.4 are unchanged.
+- **`<x-navigation-rail>`** publishes its value as `--md-navigation-rail-value`, `collapsed` or
+  `expanded` (M3's two rail values). Content an application puts in a rail reads it with
+  `@container style(--md-navigation-rail-value: collapsed)` instead of copying the rail's
+  conditions from `navigation-rail.css`.
+- **`<x-loading size="96">`** sizes the loading indicator in px, 24 to 240 (M3's responsive range),
+  with the container and the shape in proportion. A width and height of the application's own
+  still work.
+
 ## From 1.x to 2.0.0
 
 2.0.0 aligns the library with Material Design 3 Expressive as Google documents it
@@ -122,8 +134,9 @@ 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. The `rail-collapsed:` variant is gone: style a rail's collapsed shape in the
-  application's CSS with the conditions `resources/css/components/navigation-rail.css` lists. `data-app-shell`, `data-app-shell-bar`, `data-app-shell-actions` and
+  and `fill` props. The `rail-collapsed:` variant is gone: style what the application puts in a
+  rail by the rail's value, `@container style(--md-navigation-rail-value: collapsed)` (2.1.0).
+  `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