Fix mask interpretation: treat value 0 as background, not Water

Fix spatial size mismatch for patch sizes larger than image dimensions

Add per-class pixel distribution to diagnostics, raise patch slider to 512

Fix patch extraction to always sample image edges

Robust mask remapping: auto-detect class values, add diagnostics, fix shape mismatch

Fix mask filenames: TRAINING.tif / GROUND TRUTH.tif

Switch to real dataset: 4-class, rasterio patch loader, ignore unlabeled pixels

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Delete data.py

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