feat(avalonia): P6 context menu, carousel and stepper

[Shewart]

Summary

Ports the three P6 components MAUI already has to Avalonia, with the same names and properties. Builds on the P5 branch.

• context-menu (ContextMenu, ContextMenuTrigger, ContextMenuContent, ContextMenuItem, ContextMenuSeparator): opens at the pointer on right-click, and under the trigger on the Menu key / Shift+F10 or a touch hold. A second right-click moves the open menu. Picking an item, a left-click on the trigger, a click outside or Escape closes it.
• carousel: arrows, dots, loop, drag and auto-play. The arrows are tab stops, and Left / Right move while one has focus. A drag starts after 6px, so clicks inside a slide still work.
• stepper (Stepper, StepperStep): numbered steps with Previous / Next / Confirm. Reached steps are clickable and are tab stops.
• The Avalonia demo gets a "Menus, slides and steps" section.
• Docs list the new components and how they differ from MAUI.

Differences from MAUI

• ContextMenu also opens from the keyboard while focus is inside the trigger, and its items are tab stops.
• Carousel drags with the mouse as well as touch; with AspectRatio="0", set Height.
• Stepper keeps every step in the tree and shows only the active one.

Verification

• Solution builds with 0 warnings and 0 errors; 829 tests pass; format is clean.
• Each of the three installs alone into a fresh Avalonia app and builds with 0 warnings and no MAUI code.
• Checked in the Avalonia demo on Windows, in light and dark, with pointer and key input.

Not covered

• macOS and Linux runs.