Carousel element throws STOWED_EXCEPTION during XAML layout pass

The Carousel renderer in AdaptiveCarouselRenderer.cpp has multiple unguarded code paths that cause crashes:

1. SelectionChanged handler — FlipView.SelectionChanged fires with SelectedIndex = -1 during Items().Append(). Handler calls pipsPager.SelectedPageIndex(-1) → InvalidOperationException.

2. SetFlipViewMaxHeight — Called from LayoutUpdated event (outside the Render try-catch). GetAt(SelectedIndex()) called without bounds check (SelectedIndex can be -1). try_as<FrameworkElement>() result used without null check before .Measure().

3. Render loop — try_as<FrameworkElement>() result chained directly into .LayoutUpdated() without null check.

4. InitialPage — Index not validated against Items().Size().

Watson callstack (crash point)

• Frame 0: CLayoutManager::UpdateLayout (layoutmanager.cpp:469)
• Frame 1: CCoreServices::NWDrawTree
• Frame 2: CCoreServices::NWDrawMainTree
• Frame 3: CWindowRenderTarget::Draw

Repro steps

1. Build and run source/uwp/winui3/SimpleVisualizer/SimpleVisualizer.sln
2. Click "Show Adaptive Card" (loads Clipchamp-style Carousel template)
3. App crashes with InvalidOperationException

Start in AdaptiveCarouselRenderer.cpp and trace the SelectionChanged handler, SetFlipViewMaxHeight from LayoutUpdated, the Render loop, and InitialPage validation. Build source/uwp/winui3/SimpleVisualizer/SimpleVisualizer.sln and reproduce the carousel flow; done means the listed invalid selections, null elements, and out-of-range initial index no longer crash layout.