Fix odd-grid RFFT energy weights; add a Hermite boundary diagnostic

[rogeriojorge]

Problem

The field-energy diagnostic gave the last stored rFFT coefficient weight 1 for every Nx. That is right only for even Nx (Nyquist singleton); for odd Nx that coefficient has an omitted conjugate and needs weight 2. On main, EM_energy / (0.5 * mean(F^2)) = 0.898 for a random field at Nx=7 and 0.809 at (Ny,Nx,Nz)=(3,5,4); exactly 1 for even Nx. A Landau run at Nx=3 reports exactly half the field energy of the same run at Nx=4.

Change

• _rfft_weights(Nx, Nx_kept): k=0 weight 1, Nyquist weight 1 only for even Nx, others 2. It requires the original Nx (no guess from the stored length) and raises if the stored axis is not a full rfft (Nx_kept != Nx//2+1, e.g. cropped/dealiased arrays). simulation passes the static Nx to diagnostics.
• hermite_tail_fraction(output, width=1): per-species fraction of the Parseval-weighted dCk norm in the outer width modes of each active Hermite axis, shape (Nt, Ns). It is a coefficient-norm indicator, not an energy.

Tests (tests/test_diagnostics.py, 22 cases)

Independent Parseval: EM_energy vs numpy full fftn sum and real-space mean for Nx in {1,2,3,4,5,6,7,8,9} with full-FFT y/z axes and singleton axes; refusal of cropped axes and of missing Nx; tail fraction with odd weights, 3 species with unequal tails, mixed active axes, zero norm.

Evidence (tested SHA 6eeebb8, rebased on main ab87385; local, Python 3.12.9, JAX 0.11.2)

python -m pytest -n 2 -q tests: 30 passed (main: 8).

No dependencies. Overlaps textually with #13 only in _simulation.py output assembly (trivial).

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
spectrax/_diagnostics.py	94.59%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
spectrax/_simulation.py	100.00% <100.00%> (ø)
spectrax/_diagnostics.py	83.47% <94.59%> (+9.60%)	⬆️

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