Crank–Nicolson test on circle (periodic).

Files:
- cn_circle.py: CN solver + exact Fourier reference; exports CSV.
- results_cn_circle.csv: grid results (N, Nt, time_s, relative L2 error).
- plots/: optional (graphs if needed).

How to run:
1) activate venv (if used)
2) python cn_circle.py
Output: results_cn_circle.csv

Environment:
Python 3.x, numpy 2.4.2