Phase 1 of CalendarView WinUI 3 parity upgrade

Parent tracking issue: (will be linked from epic)

Scope

Update CalendarView_themeresources.xaml from legacy SystemControl* brushes to modern Fluent Design tokens. Add all missing resource keys. Add the CalendarViewBaseItemRoundedChromeEnabled boolean resource flag.

Missing resources to add

Today variants:

• CalendarViewTodayBackground, CalendarViewTodayHoverBackground, CalendarViewTodayPressedBackground, CalendarViewTodayDisabledBackground
• CalendarViewTodayBlackoutBackground, CalendarViewTodayBlackoutForeground
• CalendarViewTodaySelectedInnerBorderBrush

Blackout:

• CalendarViewBlackoutBackground, CalendarViewBlackoutStrikethroughForeground

Interaction states:

• CalendarViewSelectedHoverForeground, CalendarViewSelectedPressedForeground, CalendarViewSelectedDisabledForeground, CalendarViewSelectedDisabledBorderBrush
• CalendarViewOutOfScopeHoverForeground, CalendarViewOutOfScopePressedForeground
• CalendarViewCalendarItemHoverBackground, CalendarViewCalendarItemPressedBackground, CalendarViewCalendarItemDisabledBackground, CalendarViewCalendarItemBorderBrush

Navigation:

• CalendarViewNavigationButtonBackgroundPointerOver, CalendarViewNavigationButtonBackgroundPressed
• CalendarViewHeaderNavigationButtonForeground* variants

Layout metrics:

• CalendarViewDayItemFontSize, CalendarViewMonthYearItemFontSize
• CalendarViewDayItemMargin, CalendarViewWeekDayMargin, CalendarViewWeekDayPadding, CalendarViewMonthYearItemMargin
• CalendarViewFirstOfMonthLabelFontSize, CalendarViewFirstOfYearDecadeLabelFontSize

Font weights:

• CalendarViewHeaderNavigationFontWeight, CalendarViewWeekDayFontWeight, CalendarViewTodayFontWeight

Feature flag:

• CalendarViewBaseItemRoundedChromeEnabled (Boolean, default false initially)

Files to modify

• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarView_themeresources.xaml
• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarView.h.cs (add new internal brush/property members)
• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarView_Partial.cs (constructor brush initialization)

WinUI source reference

• controls/dev/CommonStyles/CalendarView_themeresources.xaml
• dxaml/xcp/core/core/elements/CalendarView.cpp (theme resource setup)

Dependencies

None (first phase)

Risk & Testing

• Risk: LOW - purely additive, existing keys unchanged
• Size: Medium PR (~200-300 lines XAML, ~50 lines C#)
• Testing: Visual regression via SamplesApp CalendarView_Theming sample; verify identical rendering before/after