feat(avalonia): P4 feedback components

[Shewart]

Ports the P4 tier to Avalonia: spinner, tooltip, toast, alert-dialog and hover-card.

Changes

• Spinner: Sm / Default / Lg, any token, spins only while on screen
• Tooltip: hover with a delay, also on keyboard focus; Placement uses Avalonia's PlacementMode (Top, Bottom, Left, Right) and keeps its 6px gap when flipped
• Toast: Toast.Show / Success / Error / Warning / Info / Dismiss; the stack lives in the window's overlay layer above dialogs, pauses on hover, has an optional action button and a close button on hover; <ui:Toaster Position MaxVisible> is optional
• AlertDialog: ShowAsync() returns the choice, Confirmed / Cancelled events, backdrop doesn't dismiss, Escape = Cancel, focus trapped with Cancel focused first; extra XAML children go between the description and the buttons
• HoverCard (+ trigger and content): opens on mouse hover; touch, pen and Enter / Space toggle it. A card opened by hover has no light dismiss (Avalonia's window-wide dismiss layer took the pointer off the trigger and made it flicker); one opened by a tap closes on a tap outside, and that click still reaches its target
• ShellFloatingHost.ClaimsChild: lets a host place XAML children itself (used by AlertDialog)
• ShellLabel wraps by default like MAUI's Label (TextWrapping="NoWrap" for one line)
• Demo: "Feedback" section
• Docs: COMPONENTS.md Avalonia section and differences table, plan updated

Verification

• Solution and Avalonia demo build 0 warnings / 0 errors; 793 tests pass; dotnet format check clean
• Each P4 component, plus label and dialog, installed alone into a fresh dotnet new avalonia.app builds 0/0 with no MAUI code
• Demo checked in light and dark: spinners, stacked toasts above the dialog backdrop that dismiss themselves, tooltip placement and gap, hover card text wraps, alert dialog focuses Cancel
• Hover card and tooltips checked with a real mouse: stable while hovered, close after leaving