Fix PyTorch backend quantile axis handling

[FlorianPfaff]

Summary

• wrap the exposed PyTorch backend quantile with a NumPy-compatible signature
• support axis, keepdims, out, method, and deprecated interpolation arguments when using pyrecest.backend.quantile
• add a PyTorch subprocess regression test for NumPy-style axis= and keepdims= usage

Rationale

joint_registration_assignment() initializes without an explicit transform via backend.quantile(..., axis=0). Under the PyTorch backend, pyrecest.backend.quantile currently exposes raw torch.quantile, which uses dim= rather than NumPy's axis=. That makes otherwise portable registration code fail only when PYRECEST_BACKEND=pytorch.

Validation

• Syntax-compiled the modified backend file and new regression test in the execution sandbox.
• Locally smoke-tested the corresponding torch.quantile(..., dim=..., keepdim=...) calls against installed PyTorch 2.10.0+cpu.
• Direct repository checkout/pytest execution was not possible because the execution sandbox cannot resolve github.com; PR CI should run the focused backend-portability test and full matrix.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	43.29s
✅ JSON	prettier	7	0	0	0	1.26s
✅ JSON	v8r	7		0	0	4.72s
⚠️ MARKDOWN	markdownlint	66	0	1	0	1.81s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.58s
✅ PYTHON	black	756	0	0	0	38.39s
✅ PYTHON	isort	756	0	0	0	2.66s
✅ REPOSITORY	checkov	yes		no	no	45.5s
✅ REPOSITORY	gitleaks	yes		no	no	17.57s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	21.55s
✅ REPOSITORY	syft	yes		no	no	5.95s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.98s
✅ REPOSITORY	trufflehog	yes		no	no	22.55s
✅ YAML	prettier	10	0	0	0	0.72s
✅ YAML	v8r	10		0	0	11.62s
✅ YAML	yamllint	10		0	0	0.56s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 1 error

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]

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