Improve macOS drag preview window smoothness and stabilize size/offset during tab drag

[danwalmsley]

Summary
This PR improves the drag preview behavior used when dragging document tabs out into a floating window, with a focus on macOS where the preview window was jittery and could resize/shift unexpectedly.

What changed
Coalesce drag preview window position updates on macOS using Dispatcher.UIThread.Post(..., DispatcherPriority.Render) instead of updating position on every pointer event.
Add stale callback/session guards so older scheduled move callbacks are ignored after newer ones are queued.
Skip no-op move posts when the target position/status is unchanged (or already pending unchanged).
Preserve preview pointer offset near monitor edges by removing the custom “fit inside working area” clamp.
Freeze drag preview window size once dragging is active (SizeToContent = Manual) to avoid resize jumps.
Maintain the frozen preview size across monitor/DPI changes by storing frozen dimensions in physical pixels and recalculating DIP sizes when RenderScaling changes.
Reduce unnecessary managed-overlay preview remeasure work by only remeasuring when preview status actually changes.
Why
On macOS, moving a transparent top-level preview window is more prone to jitter than on Windows. These changes reduce scheduler churn and visual corrections, and prevent preview resizing/offset changes during cross-window/cross-monitor drags.

Scope
DragPreviewHelper.cs (main implementation changes)
Validation
Built successfully with:
Dock.Avalonia.csproj -c Debug
Notes
This significantly improves smoothness, but a small amount of residual macOS jitter may remain due to native transparent window movement/compositor behavior. A future architectural step (overlay-based ghost within the app window) would likely improve this further.