[2.7] Fourier-ratio deconvolution, thickness map, residual view

The remaining eXSpy gallery workflows:

- Fourier-ratio deconvolution (needs a low-loss spectrum — handle the pairing explicitly).
- Thickness (t/λ) map.
- Residual view for judging fit quality, following the gallery's "Plot Residual" example.

Part of #60.

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Plan: [`RELEASE_0_3_0_PLAN.md`](../blob/main/RELEASE_0_3_0_PLAN.md)