Morph the rail's FAB, and offer a divider, no fill and a width floor

M3 asks the rail to animate its FAB into an extended FAB as it expands; the
documented pattern was two FABs swapped by `display`, so the collapsed one
appeared at once while the rail's width was still springing. The header now takes
one <x-fab label icon> whose label springs open and shut on the rail's own
spring — and names it at both widths, where the collapsed FAB had needed a
tooltip to have a name at all.

M3 also lists two container treatments as optional that the library had neither
of: `divider` draws the vertical divider on the page's edge, which is its own
answer to a page scrolling under a fixed rail, and `fill="false"` drops the
container colour. Neither reaches a rail open over a scrim. And a `collapsible`
rail, whose width is the visitor's choice rather than the window's, is held to
the collapsed 96px below `medium`, where M3 asks for a navigation bar instead.
Plan: docs/plans/material-3-alignment.md, step 21 (navigation N-22, N-23, N-24).

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

resources/views/showcase/shell.blade.php

@@ -45,8 +45,7 @@
             </x-slot:brand>
 
             <x-slot:rail-header>
-                <span class="rail-collapsed:hidden"><x-livewire-material::fab label="Compose" icon="edit" x-on:click="materialToast('Compose opens here')" /></span>
-                <span class="hidden rail-collapsed:inline-flex"><x-livewire-material::fab icon="edit" tooltip-right="Compose" x-on:click="materialToast('Compose opens here')" /></span>
+                <x-livewire-material::fab label="Compose" icon="edit" x-on:click="materialToast('Compose opens here')" />
             </x-slot:rail-header>
 
             <x-slot:rail-footer>