Let the navigation bar hide on a scroll down

Plan step 25 (navigation Missing): M3's "hides on scroll-down, reappears on
scroll-up" had no implementation. `<x-navigation-bar hide-on-scroll>` slides the
bar out on the default spatial spring — an instant swap under reduced motion,
which zeroes the duration token — and never while a snackbar, a bottom sheet or
a drawer is anchored to its edge; focus reaching the bar brings it back.
`<x-app-shell hide-bar-on-scroll>` picks it, and --material-bottom-bar follows
the bar down and up so a `fab` button and the snackbar keep their offsets.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

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

@@ -718,7 +718,7 @@ A visitor who has pressed the menu button keeps that choice in both standard ban
 ```
 
 - `destinations`: `title`, `icon`, `url`; optional `active` (default: the URL is the page's, also during a Livewire update request), `badge` (`true` for a dot, or a count), `badgeLabel` (what a screen reader hears for the badge: "3 unread"), `section` (a heading in the rail, shown only while it is expanded; consecutive destinations with the same section are grouped), `bar` (default `true`; `false` keeps it out of the bottom bar — M3 wants three to five there), `navigate` (`false` for a full page load instead of `wire:navigate`).
-- Slots, each rendered once: `banner` (a bar across the whole window, above the rail and the page), `brand` (beside the rail's menu button, expanded only), `rail-header` (a FAB), `rail-footer` (pinned to the foot of the rail), `actions` (a row of icon buttons at the very foot, stacked when collapsed), `top` (the page's own bar, above the page and beside the rail), and the page. `label` names the landmarks ("Main"); `rail-width` is the expanded width (`16rem`); `tall-bar` picks M3's 80px navigation bar over the 64px one.
+- Slots, each rendered once: `banner` (a bar across the whole window, above the rail and the page), `brand` (beside the rail's menu button, expanded only), `rail-header` (a FAB), `rail-footer` (pinned to the foot of the rail), `actions` (a row of icon buttons at the very foot, stacked when collapsed), `top` (the page's own bar, above the page and beside the rail), and the page. `label` names the landmarks ("Main"); `rail-width` is the expanded width (`16rem`); `tall-bar` picks M3's 80px navigation bar over the 64px one; `hide-bar-on-scroll` lets the bar leave the window while the page scrolls down.
 - `banner` or `top`: M3's scaffold is bars, then rails, then panes. An application-wide bar — one search, one account menu, the same on every page — goes in `banner` and the rail starts under it; a bar that titles the page goes in `top`, beside the rail. Never both. A banner that pins itself to the top of the window says how tall it is (`style="--material-banner: 4rem"` on `<x-app-shell>`), so the rail sticks under it instead of behind it.
 - The rail is one element at every width: what is in it is also what a phone sees in the modal rail. On a compact window nothing opens it but `$store.rail.show()`, so a page whose destinations are not all in the bar needs a menu button in its app bar (hidden from `medium`).
 - `--material-margin` is M3's window margin (16px compact, 24px from `medium`) and the content region already carries it, so a page inside the shell writes no gutters of its own; something that must reach the window's edges opts out with `-mx-(--material-margin)`.
@@ -739,7 +739,7 @@ M3 Expressive's flexible navigation bar, for three to five destinations. It does
 </div>
 ```
 
-64px in surface-container with the bottom safe area under it. Narrower than 600px the icon sits in a 56×32 indicator over the label; from 600px (the bar's own width) icon and label share a 40px pill and the items gather in the middle. `<x-navigation-bar>`: `label` ("Main"), `tall` (M3's 80px container, which keeps the icon over the label at every width — `<x-app-shell tall-bar>` picks it, and the bottom offset grows with it). `<x-navigation-bar-item>`: `label` / slot, `icon`, `link` (with `wire:navigate` unless `external` or `no-wire-navigate`; without a link it is a button), `active` (`aria-current="page"`, filled icon, secondary-container indicator), `badge` (`true` for a dot, a number for a count, 999+ at most), `badge-label` (what a screen reader hears instead of ", 3").
+64px in surface-container with the bottom safe area under it. Narrower than 600px the icon sits in a 56×32 indicator over the label; from 600px (the bar's own width) icon and label share a 40px pill and the items gather in the middle. `<x-navigation-bar>`: `label` ("Main"), `tall` (M3's 80px container, which keeps the icon over the label at every width — `<x-app-shell tall-bar>` picks it, and the bottom offset grows with it), `hide-on-scroll` (M3's scrolling behaviour: the bar slides out on a scroll down and springs back on a scroll up, never before the first screenful and never while a snackbar, bottom sheet or drawer is on screen; focus reaching it brings it back. `<x-app-shell hide-bar-on-scroll>` picks it, and `--material-bottom-bar` goes down and comes back with the bar, so a `fab` button and the snackbar keep their distance from it). `<x-navigation-bar-item>`: `label` / slot, `icon`, `link` (with `wire:navigate` unless `external` or `no-wire-navigate`; without a link it is a button), `active` (`aria-current="page"`, filled icon, secondary-container indicator), `badge` (`true` for a dot, a number for a count, 999+ at most), `badge-label` (what a screen reader hears instead of ", 3").
 
 ### `<x-navigation-rail>`, `<x-navigation-rail-item>`, `<x-navigation-rail-section>`