Fix odd-grid RFFT energy weights

[rogeriojorge]

Summary

• weight the last RFFT coefficient as a Nyquist singleton only when Nx is even
• preserve the DC-only Nx=1 case
• test DC, even-Nyquist, and odd-grid conjugate-pair energies

Root cause

The electromagnetic-energy diagnostic always assigned weight one to the final RFFT coefficient. That is correct for an even grid, where the final coefficient is the Nyquist singleton. For odd Nx, the final retained coefficient still has an omitted negative-frequency conjugate and must have weight two.

Impact

Electromagnetic and total-energy diagnostics now satisfy Parseval weighting for both odd and even Fourier resolutions. This is especially relevant because the supplied SPECTRAX examples commonly use odd Nx values.

Validation

• python3 -m pytest -q — 6 passed
• python3 -m flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics — 0
• physical-space versus RFFT field energy for Nx = 1, 4, 5, 32, 33 — agreement to 7e-16 or better

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
spectrax/_diagnostics.py	74.44% <100.00%> (+0.58%)	⬆️

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