Add saturation-aware analysis to birefringence optimization

The birefringence optimization workflow now detects and reports pixel
saturation in source images during paired acquisition. When birefringent
tissue pushes pixel values to the detector ceiling, I(+)-I(-) is
underestimated, potentially recommending incorrect angles.

Changes:
- Track per-angle saturation fraction on both +theta and -theta images
- Saturation-aware optimal angle selection with non-saturated alternative
- Red X markers on saturated angles in visualization plots
- Saturation analysis section in summary, JSON, and CSV output
- Sat% column in results tables with * markers for saturated angles
- Warnings and remediation recommendations when saturation detected

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MichaelSNelson