Implement a more robust scoring mechanism for screw optimization and
add functionality to generate synthetic X-ray projections (AP and
lateral views) from CT data.
Key changes:
- core: add `generate_cylinder_butt_torch` to create a mask for the
screw entrance (0.25mm) to exempt it from bone-breaching penalties.
- core: update `cl_score_torch_xfr` to include a diameter preference
bonus and utilize the entrance mask.
- core: adjust optimizer bounds and scoring weights to favor larger
diameter screws and improve convergence.
- xfr_cbt_native: implement `render_xray_projections` to generate
synthetic AP and lateral X-ray images for visualization.
- visualization: enhance `render_bone_figure` with semi-transparent
spinous process rendering and improved depth sorting for screws.
- xfr_debug: improve level detection to support arbitrary lumbar
levels (L1-L9) and add safe volume-level cleanup for CBT writing.
- config: update allowed diameters and lengths constants.
