Correct numerical findings and add reproduction tools

[osaether]

The numerical findings prescribed an ambiguous far-field threshold, an incomplete lossy-line calculation, and a skin-depth warning that could miss under-resolved elements. Correct those recommendations and distinguish mesh sensitivity from demonstrated error against a converged reference.

Add a C cancellation harness and a standard-library Python script that reproduce the frequency and mesh tables using temporary inputs. The calculator's runtime behavior is unchanged.

Validation:

• All 123 pytest tests pass.
• C harness compiled with GCC -O2 -Wall and reproduced the cancellation table.
• Python diagnostic reproduced all frequency and mesh table values at the reported precision.
• git diff --check passed.