feat: Add multithreaded Real-Time Filter Preview

[PLakhmale]

Summary

This PR engineers a new threaded infrastructure to support real-time filter previews within the FilterPanel. Previously, modifying a JSpinner or threshold slider forced the user to manually trigger the filter cycle. This introduces a non-blocking UI implementation that applies filters live as the user types or adjusts parameters.

Architecture & Implementation

To prevent the rapid mutation of JSpinner components from freezing the Swing Event Dispatch Thread (EDT) or exhausting system compute resources, I implemented the following architecture:

• PreviewManager Engine: A centralized thread controller extending SwingWorker<Void, Void>. Backgrounds all heavy 2D convolution matrix runs off the EDT.
• Event Debouncing: Injected a standard javax.swing.Timer (250ms) into the ChangeListener infrastructure. Rapid spinner clicks do not trigger a filter pass until the user pauses holding the UI component.
• Non-destructive Cloning: Hardpatched the Gauss2D_Filter_ pipeline to skip physical disk I/O when the filterPath parameter is explicitly null, utilizing ImageProcessor.duplicate() to run math entirely in memory for instant feedback.

GSoC 2026 Context

I engineered this specifically to address the GSoC 2026 Ideas List requirements for the "Real-Time Preview" project track. This PR serves as a functional, structurally complete prototype of my proposed architecture.

If accepted for the summer, my battle-plan is to scale this PreviewManager to seamlessly support multi-slice 3D ImageStacks and systematically patch the applyFilter logic across the remaining 14 filters to handle non-destructive memory-only modes.