Fix the Shift-drag alignment guide anchoring to the prior selection instead of the dragged layer

[Keavon]

Closes #4248

[gemini-code-assist]

Code Review

This pull request refactors how the drag start center is tracked and calculated in the select tool. The static drag_start_center field has been removed from SelectToolData in favor of a dynamically computed dragging_layers_center method. This method calculates the center of the combined viewport-space bounding box of the currently dragged layers. The FSM logic has been updated to use this dynamic calculation, adjusting for drag displacement to keep the guide anchored correctly during viewport auto-panning. There are no review comments, so no feedback is provided.

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

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