Breakage-Aware clDice

Adding breakage-aware clDice improved small and medium artery performance in the ablation. The breakage-aware clDice term uses group convolution to compute local neighborhood sums for each class in predictions and ground truth. Breakage-aware clDice avoids expensive per-class skeletonization by using a unified foreground soft skeleton. The method uses an L1 neighborhood similarity error map to highlight breakages and false positives while reducing emphasis on minor boundary mismatch.