feat(avalonia): P2 form controls : textarea, select, slider, radio group, calendar, date and time pickers

[Shewart]

Summary

Third Avalonia slice of Phase 2: the P2 form controls.

shell

• ShellAnchoredPopup: the floating-panel logic the popover host had (overlay-layer Popup under an anchor, click outside and Escape close it, one open at a time, open/close motion), now shared; ShellPopoverHost delegates to it
• ShellPlatform.StripNativeChrome(TextBox): the Fluent chrome clearing Input did inline, shared with Textarea (as on MAUI)

Components

• textarea, select, slider, radio-group (+ item), calendar, date-picker, time-picker, with the MAUI names, properties and sizes
• Select, DatePicker and TimePicker float their panel with ShellAnchoredPopup; the select list matches the trigger's width
• Slider is custom-drawn in shadcn's style (Fluent's thumb can't be made to match) and adds Step for the arrow keys

Verification

• dotnet build ShellUI.Native.slnx: 0 warnings; tests 705/705; each commit builds the Avalonia demo and passes the tests
• Each P2 component installs alone into a fresh dotnet new avalonia.app and builds with 0 warnings; all 46 Avalonia entries together build
• Demo: values apply from code; the select list matches its trigger and flips above it; the date picker opens on today; the time picker scrolls to the current hour; dropdown pick and popover Escape still work on the shared popup
• Not verified: real pointer/keyboard input (slider drag included), Linux

Differences from MAUI

• Slider ValueChanged passes the new double; DatePicker DateChanged passes the new DateTime