feat: bottom-nav, a mobile tab bar for MAUI and Avalonia

[Shewart]

Description

bottom-nav: a mobile tab bar for MAUI and Avalonia (BottomNav + BottomNavItem, shellui-native add bottom-nav).

Stacked on the headless tests PR (base test/avalonia-headless); this PR adds one commit.

• Variants:
  • Default: a line slides to the active item; ActiveToken colors it.
  • Pill: Material 3, a pill grows behind the active icon.
  • Floating: an inset rounded bar; a pill slides behind the active icon, the label under it.
  • Lifted: the active icon rises into a ringed circle over the bar's top edge; the circle slides between items.
  • Expanding: a compact capsule; the active item widens into a pill with its label.
• Labels: Always, Active (the other items center their icon) or Never.
• Items: Badge, Dot, ActiveIcon, IconContent (e.g. an Avatar), Prominent (a round action button that rises over a full-width bar, raises Clicked and is never selected).
• ScrollSource / IsHidden: scrolling down slides the bar away, scrolling up brings it back.
• BottomInset: by default the safe area under the bar (Android, iOS, Avalonia edge-to-edge windows); a number overrides it.
• SelectedValueChanged reports user input only. Avalonia items are tab stops; Left / Right move along the bar.
• About 200 ms transitions and a small press-in on the icon.
• Also: demo sections on both platforms, the CLI template and registry entry, docs/COMPONENTS.md and the roadmap, headless tests.

Type of Change

• New component (adding a new UI component)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added comments where necessary
• I have updated the documentation (if applicable)
• My changes generate no new warnings
• I have tested on at least one platform (MAUI/WinUI/WPF)

Testing Done

• Solution builds with 0 warnings; 1473 main tests and 163 headless tests pass; format clean.
• Headless tests: for every variant, values set from code raise no event, a tap raises it once, the prominent item is not selected, Right skips it; hide on scroll; 0 inset on the desktop.
• MAUI demo builds for Windows, Android and iOS.
• add bottom-nav builds 0/0 in a fresh Avalonia app and in a fresh dotnet new maui app (Android and Windows).
• Checked in light and dark: Avalonia (headless renders) and the MAUI demo on Windows, including selection changes and hide on scroll.