Preserve custom distribution subclasses in conversion factories

[FlorianPfaff]

Summary

Fix five custom-distribution conversion factories so the target-centric convert_distribution(source, TargetSubclass) API returns the requested subclass instead of silently constructing the base representation.

Affected targets:

• CustomLinearDistribution
• CustomHypercylindricalDistribution
• CustomHypersphericalDistribution
• CustomHyperhemisphericalDistribution
• CustomHemisphericalDistribution

Root cause

convert_distribution resolves TargetSubclass.from_distribution(...). These inherited factories were staticmethods and explicitly constructed their base class, so subclass-based extensions lost their concrete target type. This is the same factory-contract issue fixed for several Dirac/grid targets in #5347, but the custom-distribution factories were still affected.

Fix

• convert the five factories to classmethods
• construct results through cls(...)
• preserve the existing normalization/scaling branches and all density mathematics
• add regression coverage through the public convert_distribution gateway for all five target families

Validation

• branch is based directly on current main at baac3b1736bb8e81eafc1e3f69e10104636103b9
• final comparison: 6 files changed, 6 commits ahead, 0 behind
• exact patched production files and the new regression test syntax-compile successfully locally
• a full local pytest run is unavailable in this execution environment because PyRecEst is not installed and external cloning is unavailable; GitHub Actions is the authoritative repository-wide validation

[github-actions]

✅MegaLinter analysis: Success

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✅ JSON	v8r	7		0		0	4.09s
✅ MARKDOWN	markdownlint	68	0	0		0	2.64s
✅ MARKDOWN	markdown-table-formatter	68	0	0		0	0.72s
✅ PYTHON	black	1989	15	0		0	64.7s
✅ PYTHON	isort	1989	27	0		0	2.87s
✅ REPOSITORY	betterleaks	yes		no		no	1.71s
✅ REPOSITORY	checkov	yes		no		no	33.31s
✅ REPOSITORY	git_diff	yes		no		no	0.14s
✅ REPOSITORY	secretlint	yes		no		no	104.78s
✅ REPOSITORY	syft	yes		no		no	2.7s
✅ REPOSITORY	trivy-sbom	yes		no		no	2.5s
✅ YAML	prettier	11	0	0		0	0.69s
✅ YAML	v8r	11		0		0	17.28s
✅ YAML	yamllint	11		0		0	1.0s

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