Hide a disabled fab button on a compact window, as M3 removes a FAB

`<x-button fab>` is an extended FAB below `medium` and a filled button
from there, but a disabled one was still drawn as the FAB below
`medium`: the compact FAB's rule set no state of its own, so the button
stood greyed out and fixed over the content. M3 never disables a FAB:
"if its action is unavailable, remove the FAB entirely".

A `fab` given `disabled` (a `<button disabled>`, or a link's
`aria-disabled="true"`) now renders `data-md-unavailable`, and
button.css draws it `display: none` below `medium`, which also takes it
out of the accessibility tree and the Tab order; from `medium` it is
the disabled filled button it was. The mark comes from the prop, not
from `:disabled`, because a `spinner` puts `disabled` on its button
while the action runs, and that FAB is busy rather than unavailable:
hidden, it would vanish instead of showing its loading indicator. The
docblocks and the skill's button table say so.

A browser test renders an enabled fab with a slow spinner action, a
disabled one and a disabled link on a 393px window: the disabled two
are not rendered, the busy one stays on screen with its indicator, and
at 600px both disabled ones are the disabled button again; it fails
without the change in Chrome, Firefox and Safari (and in Chrome with
`:disabled` alone as the condition, on the busy FAB). A feature test
reads the mark and the rule.

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

UPGRADE.md

@@ -74,6 +74,13 @@
   `<x-checkbox>` did the same with its box. Both roots now keep their automatic minimum — the
   control, and beside a label its longest word — so the text wraps instead. A `flex: none` an
   application put on the switch to hold it can go.
+- **A disabled `<x-button fab>` is not drawn on a compact window.** Below `medium` it was still
+  the extended FAB, greyed out and fixed over the content; M3 never shows a disabled FAB and
+  removes one whose action is unavailable, so a `fab` given `disabled` (on a link too) now renders
+  `data-md-unavailable` and is `display: none` there, out of the accessibility tree with it. From
+  `medium` it is the disabled filled button, as before. A `spinner` that disables the button while
+  its action runs leaves the FAB on screen. An application's own `hide-below` or condition around
+  a disabled fab button can go.
 
 ## From 2.0.0 to 2.1.0