feat(avalonia): port the remaining 14 components

[Shewart]

Summary

Ports the 14 components that were still MAUI-only to Avalonia, with the same names and properties. Every component in the registry now has an Avalonia version.

• Form controls: toggle-group, number-input, tag-input, input-otp, combobox, multi-select.
• Data display and layout: kbd, copy-button, stat-card, link-card, tree-view, timeline, aspect-ratio, wrap-layout.
• The Avalonia demo gets "More form controls" and "More data display" sections.
• Docs: the Avalonia section now says every component is ported, lists how the new ones differ from MAUI, and notes how to handle names that match Avalonia's own controls.

Differences from MAUI

• Toggle-group items and tree rows are tab stops; in the tree, Right expands and Left collapses.
• Number input steps with Up / Down while the field has focus.
• Tag input removes the last tag on Backspace in the empty field.
• Combobox and multi-select focus the search field on open; Up / Down move a highlight that Enter takes.
• Copy button and link card use the window's clipboard and launcher.
• Aspect ratio is a Border, so CornerRadius rounds the content.
• Wrap layout is a Panel without Padding.

Verification

• Solution builds with 0 warnings and 0 errors; 885 tests pass; format is clean.
• Each of the 14 installs alone into a fresh Avalonia app and builds with 0 warnings and no MAUI code.
• Checked in the Avalonia demo on Windows, in dark and light, with pointer and key input.

Not covered

• The copy button's clipboard write and a link card opening its Url were not exercised.
• macOS, Linux and touch.
• The Avalonia templates are not on NuGet yet; they ship with the next release.