Fix periodic Orszag-Tang grid

[rogeriojorge]

Summary

• use jnp.linspace(..., endpoint=False) for the three periodic Orszag-Tang coordinates
• keep the fix local to the example; no new helper API or tests

Root cause

The example included both endpoints of each periodic interval. The Fourier solver expects N distinct nodes with spacing L / N, so duplicating the endpoint injected unintended Fourier content into the initial condition.

Impact

The Orszag-Tang initialization now matches SPECTRAX's discrete periodic Fourier grid with a three-line change.

Validation

• pytest -q - 3 passed
• git diff --check - passed
• isolated Fourier-mode leakage on a 33-point grid: 1.42e-16

[codecov]

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