- Implement apply_stable_cascade_qr_filter() using kornia HSV conversion
- Add filter to artistic pipeline only (prevents grayscale output)
- Use 3-level brightness quantization (0.0, 0.5, 1.0) for better ControlNet perception
- Apply Gaussian blur (kernel=15) and adaptive thresholding (33% rule)
- Add UI checkbox in artistic settings (disabled by default)
- Fix parameter naming: rename 'progress' to 'gr_progress' throughout
- Add enable_cascade_filter parameter to _pipeline_artistic() function

Filter improves QR scannability by providing better brightness guidance to the
brightness ControlNet while maintaining color output through FreeU/SAG enhancements.