Fix: Reduce selection lag during drag selection by throttling SelectionChanged events

[LeoAndJellyfish]

Resolved / Related Issues

• Closes Bug: Lag when drag selecting multiple items due to frequent SelectionChanged events #18289

Summary of Changes

Introduced a selection update timer to reduce the frequency of SelectionChanged events during drag selection, improving performance through batched selection updates.

Key Changes:

• Added selectionUpdateTimer with 8ms interval (~120fps)
• Added pendingSelectedItems and pendingRemovedItems lists to buffer pending changes
• Unified selection state updates when the timer triggers
• Ensured correct final selection state when pointer is released
• Accumulated removed items to prevent state loss during rapid dragging

Technical Details:

• Uses DispatcherQueueTimer for 8ms throttling interval
• Accumulates pendingRemovedItems to ensure all selection changes are correctly processed during rapid dragging
• Batch triggers SelectionChanged event in timer callback

Steps used to test these changes

1. Opened Files and navigated to a folder with many files (100+)
2. Performed drag selection (box selection) across multiple files
3. Verified smooth selection updates during drag
4. Verified correct final selection state after releasing pointer
5. Tested rapid drag selection to ensure no selection state is lost
6. Tested with Ctrl+drag (invert selection) and Shift+drag (extend selection)