Fixes FluidMoveBehavior static dictionary memory leaks

[Brian Lagunas (brianlagunas)]

Description of Change

Addresses memory leaks in FluidMoveBehavior's static TagDictionary and TransitionStoryboardDictionary. Previously, these dictionaries could hold strong references to UIElements and Storyboards even after the associated elements were removed from the visual tree or garbage collected, leading to memory accumulation over time.

This change introduces several improvements:

• The TagData structure now uses WeakReference for Child and Parent properties, allowing elements to be garbage collected when no other strong references exist.
• Implements a throttled purge mechanism in OnLayoutUpdated to regularly clean up TagDictionary entries whose associated child elements are no longer alive or are unloaded from the visual tree.
• Adds a PurgeDeadStoryboards method to proactively remove Storyboard entries from TransitionStoryboardDictionary if their corresponding FrameworkElement key is unloaded, if the associated TagData's child is dead, or if the TagDictionary entry is missing.
• Incorporates eager removal of dead entries within UpdateTagData and UpdateLayoutTransitionCore to prevent re-adding already-collected elements.
• Ensures Storyboards are explicitly stopped and removed when a FluidMoveBehavior is detached or its tracked children are removed from the panel.
• Includes a new comprehensive unit test suite, FluidMoveBehaviorTest.cs, to validate the weak reference behavior, dictionary purging, and overall retention logic in various scenarios (element removal, reparenting).

Note on ConditionalWeakTable: I considered replacing the static TagDictionary with ConditionalWeakTable to eliminate strong-key retention, but I don’t think it is a safe drop-in change here. While it works well for reference-identity keys like FrameworkElement and some DataContext cases, FluidMoveBehavior also supports TagPath, which means tags can be scalar values or strings and currently rely on normal Dictionary<object, ...> equality semantics (Equals / GetHashCode). ConditionalWeakTable uses reference identity, not value equality, so switching to it could break matching behavior for TagPath-based tags even when the values are logically equal. It also would not help much with transitionStoryboardDictionary, which still needs explicit enumeration and cleanup for stopping storyboards and tearing down overlay state. Because of that, I think ConditionalWeakTable is appealing for leak prevention but too risky here unless the behavior is redesigned around reference-identity-only tags.

Bugs Fixed

• FluidMoveBehavior memory leak #38 - Fixes memory leaks in FluidMoveBehavior where static dictionaries retained references to UI elements and associated storyboards after they were no longer active.

API Changes

Added:

• TagData.IsAlive property (internal)
• FluidMoveBehaviorBase.ResetPurgeThrottle() method (internal)
• FluidMoveBehavior.PurgeDeadStoryboards() method (internal)
• FluidMoveBehavior.InjectStoryboardEntry() method (internal)
• FluidMoveBehavior.StoryboardDictionaryContainsKey() method (internal)
• FluidMoveBehavior.ClearStoryboardDictionary() method (internal)

Changed:

• TagData.Child and TagData.Parent now use WeakReference internally.
• FluidMoveBehavior.OnDetaching() behavior to include storyboard cleanup.
• Internal purge logic within FluidMoveBehaviorBase.OnLayoutUpdated.

Removed:

• TagData.Timestamp property.

Behavioral Changes

Applications using FluidMoveBehavior will experience improved memory management, particularly in scenarios involving dynamic UI where elements are frequently added, removed, or reparented. Elements and their associated Storyboards will now be correctly garbage collected, preventing a steady increase in memory footprint that could occur under previous implementations.

PR Checklist

• Has tests (new FluidMoveBehaviorTest.cs file provides extensive unit tests)
• Rebased on top of master at time of PR
• Changes adhere to coding standard