Support bound Flyout opened and closed events

[wieslawsoltes]

Summary

• make EventTriggerBehavior observe Flyout.Opened and Flyout.Closed through strongly typed subscriptions
• synchronize Flyout-attached behaviors and actions with the placement target's logical/data-context scope
• preserve compiled command bindings when the Flyout is hosted outside the normal logical tree
• document the direct Flyout usage and add an end-to-end headless regression

Root cause

Avalonia 12's FlyoutBase derives from AvaloniaObject, not StyledElement. A behavior attached directly to a Flyout could register the requested CLR event through the reflection fallback, but it had no logical parent or inherited data context. Consequently, InvokeCommandAction.Command="{Binding ...}" remained unresolved and made the event appear not to fire.

The generic reflection path was also unnecessary for the two known Flyout lifecycle events.

Changes

• register Opened and Closed in AddEventHandlerRegistry through a focused FlyoutEventHandler
• avoid adding any new reflection and make these event paths trimming/AOT friendly
• observe FlyoutBase.TargetProperty
• when a placement target is assigned:
  • attach the behavior/action logical scope to that target
  • mirror the target's data context
  • update both when a Flyout is reused with another target
• dispose target and data-context subscriptions when the behavior detaches
• allow trigger actions to attach whenever their behavior has a valid logical parent
• document Flyout lifecycle event usage

Tests

A compiled-XAML headless fixture matches the reported pattern:

• behaviors are attached directly to a Flyout
• Opened and Closed each invoke a compiled-bound command exactly once
• the command binding comes from the placement button/window data context
• PassEventArgsToCommand receives the original event argument

Validation

• focused EventTriggerBehaviorTests: 5 passed
• Xaml.Behaviors.Interactivity.UnitTests: 93 passed
• Xaml.Behaviors.Interactions.UnitTests: 90 passed, 2 pre-existing skipped
• git diff --check: clean

Closes #350