Test Coverage gaps

[pmaasz]

Modules with no direct tests (only indirect / implicit):

• desensitization.py
• global_optimizer.py (GA / SA)
• mechanical_designer.py (exists as a module, no test_mechanical_designer.py)
• interactive_ray_tracer.py
• io/step_export.py (only test_step_export.py + test_step_export_multi.py — check they cover Writer, not just Exporter)

Recommended additions:

• BK7→air TIR critical angle (41.24°) — locks in §2.4.
• Thick‑lens BFL for a known doublet — locks in §2.1.
• RNG‑seeded MC that produces a pinned mean and std.
• OpticalSystem.save ↔ load round‑trip with a triplet — would have caught the duplicate to_dict/from_dict issue.