Say how the rail's FAB overrides win, and that rail-collapsed: is gone

Plan step 36, navigation review. navigation-rail.css claimed each FAB
override beats fab.css by specificity alone; the collapsed FAB's gap
and minimum width tie with fab.css's extended-size rules and win by
order, which the file's own import of fab.css guarantees. The rail
view pointed at one file for the collapsed conditions that four files
write out. UPGRADE.md now says applications lose the `rail-collapsed:`
variant the development skill used to teach.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

UPGRADE.md

@@ -106,7 +106,8 @@ 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. `data-app-shell`, `data-app-shell-bar`, `data-app-shell-actions` and
+  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
   `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