Add Hermite boundary-resolution diagnostic

[rogeriojorge]

Summary

• add hermite_tail_fraction(output, width=1) as a post-processing diagnostic
• measure the perturbation coefficient norm in the outer retained modes of every active Hermite axis, separately by time and species
• reuse the even/odd RFFT Parseval weights from Fix odd-grid RFFT energy weights #9
• leave the solver and RHS unchanged, so there is no simulation-time overhead

This is stacked on #9. After rebasing onto its simplified head, the incremental patch is 88 additions and 5 deletions across two files; the former duplicate weighting logic is now a shared three-line helper.

Physics and numerical rationale

Fourier-Hermite phase mixing transfers perturbed free energy toward high Hermite moments. A growing boundary fraction therefore warns that the velocity-space truncation is populated and recurrence or truncation error may follow. For SPECTRAX's matched-Maxwellian basis, the squared-coefficient norm is proportional to perturbed-distribution free energy; this function deliberately reports that normalized proxy rather than calling it total plasma energy.

Reference: Parker & Dellar, Fourier-Hermite spectral representation for the Vlasov-Poisson system in the weakly collisional limit, https://arxiv.org/abs/1407.1932

Validation

• pytest -q: 10 passed
• ruff check spectrax/_diagnostics.py tests/test_diagnostics.py: passed
• synthetic coverage: odd-RFFT weighting, independent species, union of active boundaries, inactive axes, zero norm, and invalid width
• two-stream, Nx=65, t=50, 501 outputs:
  • Nn=64: peak outermost fraction 2.87e-4; peak outer-eight fraction 1.66e-3
  • Nn=32: peak outermost fraction 2.46e-3; peak outer-eight fraction 1.71e-2
  • the outer-eight metric is about 10.3x larger at lower Hermite resolution

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
spectrax/_diagnostics.py	93.93%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
spectrax/_diagnostics.py	78.81% <93.93%> (+4.36%)	⬆️

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