Preserve low-rank Fourier subclasses in operations

[FlorianPfaff]

Bug

LowRankHypertoroidalFourierDistribution used hard-coded base-class constructors in several instance operations. Consequently, subclasses silently collapsed back to LowRankHypertoroidalFourierDistribution after:

• shift()
• centered_hermitianized()
• multiply()
• convolve()
• dense Fourier coercion inside binary operations

This is inconsistent with the subclass-preserving factory/operation contract used elsewhere in PyRecEst (including the dense Fourier and Fejer Fourier implementations).

Fix

Construct operation results through type(self) and convert dense operands through type(self).from_dense(...). Numerical formulas, tensor-train operations, normalization, and public signatures are unchanged.

Regression coverage

Adds a focused NumPy-only regression using a derived low-rank Fourier class. It verifies exact subclass preservation for shift, Hermitian repair, multiplication and convolution, including binary operations with a dense HypertoroidalFourierDistribution operand.

Scope

• based directly on main at baac3b1736bb8e81eafc1e3f69e10104636103b9
• 2 commits ahead, 0 behind at PR creation
• production diff: 5 additions / 5 deletions
• one 49-line regression test added
• no overlap found with the currently open low-rank/Fourier bug-fix PRs

[github-actions]

✅MegaLinter analysis: Success

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✅ PYTHON	black	1989	15	0		0	65.83s
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✅ REPOSITORY	checkov	yes		no		no	34.68s
✅ REPOSITORY	git_diff	yes		no		no	0.16s
✅ REPOSITORY	secretlint	yes		no		no	104.06s
✅ REPOSITORY	syft	yes		no		no	2.57s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.83s
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✅ YAML	yamllint	11		0		0	1.12s

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