Root Cause

When users rapidly switch tabs in TabbedPage on Windows, concurrent navigation operations cause a crash:
Stack trace shows crash at line 299 (UpdateCurrentPageContent):

WinRT.Runtime.dll!WinRT.ExceptionHelpers.ThrowExceptionForHR(int hr) Microsoft.W
inUI.dll!Microsoft.UI.Xaml.Controls.ContentPresenter.Content.set(object value) M
icrosoft.Maui.Controls.dll!Microsoft.Maui.Controls.TabbedPage.UpdateCurrentPageC
ontent(WPage page) Line 299

The Problem Flow:
1. User rapidly clicks tabs (Tab 1 → Tab 2 → Tab 3)
2. First tab change callsNavigateToPage →NavigationFrame.NavigateToType()
3. Navigation starts,OnNavigated event will fire when complete
4.Before first navigation completes, second tab change triggers anotherNavigateToPage
5. Second navigation also callsNavigateToType() → starts another navigation
6. BothOnNavigated events fire and try to setpresenter.Content
7.WinUI throws:ContentPresenter.Content cannot be modified during active navigation/layout operation

Additionally, settingSelectedItem inMapCurrentPage triggersSelectionChanged events that cause reentrant calls.

Solution

Implemented two guard flags to prevent race conditions:

1. _isNavigating flag - Prevents overlapping navigation operations
   - Set before callingNavigateToType()
   - Checked at start ofNavigateToPage() - returns early if navigation in progress
   - Cleared inOnNavigated() finally block after content update completes

2. _isUpdatingSelection flag - Prevents reentrant selection events
   - Set before updatingSelectedItem inMapCurrentPage
   - Checked at start ofOnSelectedMenuItemChanged - returns early if updating programmatically
   - Cleared in finally block after update completes

This ensures:
- Only one navigation operation runs at a time
- Subsequent rapid clicks are queued/ignored until current navigation completes
- No reentrant selection change events
- Safe cleanup via try-finally blocks

Fixes#32824