Improve graph readability and add new masking visualizations

[yuna-k1m]

• Simplified masking diagnostics into a single sorted bar chart (BPDA_PGD only)
• Removed duplicate masking_diagnostics category from heatmap functions
• Fixed x-axis scaling (auto-adjusts to data range instead of fixed 0-1)
• Fixed label rotation and clipping issues
• Added error-bar heatmap variant (omits trivial std values)
• Added scatter plot (PGD vs BPDA_PGD) to directly visualize genuine
  robustness vs gradient masking
• Added slope chart (PGD to BPDA_PGD per model)
• Added masking-severity-by-quantization-variant summary chart