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/boost/skills/livewire-material-development/SKILL.md

@@ -696,8 +696,7 @@ A visitor who has pressed the menu button keeps that choice in both standard ban
 ]">
     <x-slot:brand><a href="{{ route('home') }}" wire:navigate class="type-title-lg">SealShare</a></x-slot:brand>
     <x-slot:rail-header>
-        <span class="rail-collapsed:hidden"><x-fab label="New share" icon="add" link="{{ route('upload') }}" /></span>
-        <span class="hidden rail-collapsed:inline-flex"><x-fab icon="add" tooltip-right="New share" link="{{ route('upload') }}" /></span>
+        <x-fab label="New share" icon="add" link="{{ route('upload') }}" />
     </x-slot:rail-header>
     <x-slot:rail-footer>
         <x-navigation-rail-item label="Settings" icon="settings" link="{{ route('settings') }}" :active="request()->routeIs('settings')" />
@@ -743,7 +742,7 @@ M3 Expressive's navigation rail: collapsed (96px, icon over label) or expanded (
 <div class="flex min-h-dvh">
     <x-navigation-rail mode="collapsible">
         <x-slot:brand><span class="type-title-lg">SealShare</span></x-slot:brand>
-        <x-slot:header><x-fab icon="add" tooltip-right="New share" /></x-slot:header>
+        <x-slot:header><x-fab label="New share" icon="add" /></x-slot:header>
 
         <x-navigation-rail-item label="Shares" icon="folder_shared" link="{{ route('shares.index') }}" active badge="3" />
         <x-navigation-rail-section label="Admin">
@@ -760,8 +759,9 @@ M3 Expressive's navigation rail: collapsed (96px, icon over label) or expanded (
 ```
 
 - `mode`: `collapsed`, `expanded`, `collapsible` (default: expanded until its menu button collapses it; the choice is `$store.rail`, remembered and applied before the first paint), `modal` (collapsed in the layout; the menu button or `$store.rail.show()` opens it expanded over a scrim, focus held until Escape, the scrim or leaving the page), `adaptive` (`<x-app-shell>`'s, one rail per window size class: hidden and opened as a modal on a compact window, collapsed and opened as a modal at `medium`, a standard rail from `expanded` — collapsed there, expanded from `large`).
-- Props: `label` ("Main"), `width` (expanded width, `16rem`, held between 220 and 360px), `menu` (the menu button; on by default for `collapsible`, `modal`, `adaptive`). Slots: `brand` (beside the menu button, expanded only), `header` (a FAB), the destinations (the only part that scrolls), `footer`. In a flex row the rail sticks to the top of the viewport.
-- Anything inside a rail takes both shapes with the `rail-collapsed:` variant, true while that rail is drawn collapsed for whatever reason: `<span class="rail-collapsed:hidden">…expanded only…</span>`, `<span class="hidden rail-collapsed:inline-flex">…collapsed only…</span>`. Put the variant on a wrapper, never on a component. Nothing that shows while collapsed may be wider than 96px.
+- Props: `label` ("Main"), `width` (expanded width, `16rem`, held between 220 and 360px), `menu` (the menu button; on by default for `collapsible`, `modal`, `adaptive`), `divider` (M3's optional vertical divider on the page's side — use it when the page scrolls under a fixed rail), `fill` (`false` for a transparent container, which M3 allows while the items keep 3:1 contrast). Slots: `brand` (beside the menu button, expanded only), `header` (one `<x-fab label icon>`, which the rail morphs into an extended FAB and back as it expands — it also rests at elevation 0, as M3 asks of a nested FAB), the destinations (the only part that scrolls), `footer`. In a flex row the rail sticks to the top of the viewport.
+- Anything else inside a rail takes both shapes with the `rail-collapsed:` variant, true while that rail is drawn collapsed for whatever reason: `<span class="rail-collapsed:hidden">…expanded only…</span>`, `<span class="hidden rail-collapsed:inline-flex">…collapsed only…</span>`. Put the variant on a wrapper, never on a component. Nothing that shows while collapsed may be wider than 96px.
+- A `collapsible` rail is held to the collapsed 96px below `medium` (600px), where M3 says to use a navigation bar rather than a standard rail. `collapsed` and `expanded` are fixed-width by design: wrap one in a `medium:` element if it must not show on a phone.
 - `<x-navigation-rail-item>`: the same props as `<x-navigation-bar-item>`. `<x-navigation-rail-section label="…">`: a group with a heading that shows only while the rail is expanded; it names the group for screen readers either way.
 - `$store.rail`: `collapsed`, `toggle()`, `collapse()`, `expand()` (the remembered choice; `auto` is true while nothing is stored, so an adaptive rail takes its window size class's default instead, and the first choice clears it), `open`, `show()`, `hide()` (the modal rail; closed on every `wire:navigate`). `config/livewire-material.php` → `rail.default` (`expanded` or `collapsed`) and `rail.storage_key` (`material-rail`).