Fix JAX meshgrid input coercion

[FlorianPfaff]

Summary

• replace the direct JAX meshgrid export with a small backend wrapper that coerces lists, ranges, and scalar axes to one-dimensional JAX arrays
• preserve NumPy-style backend semantics while returning JAX arrays
• add an explicit subprocess regression test for PYRECEST_BACKEND=jax

Bug fixed

The shared backend contract accepts NumPy-style meshgrid axes such as lists, range objects, and scalar axes. jax.numpy.meshgrid rejects list/range axes and also rejects 0-D array axes, so the JAX backend could fail even though the NumPy backend passed the same contract tests.

Validation

• Local focused JAX smoke check confirmed native jax.numpy.meshgrid([0, 1], range(2), indexing="ij") raises TypeError and the wrapper produces the expected coordinate grids.
• compare_commits reports this branch is ahead of main by 2 commits and behind by 0, changing only src/pyrecest/_backend/jax/__init__.py and tests/test_backend_meshgrid_contract.py.
• Full local pytest was not run because the execution container cannot clone from GitHub; PR CI should run the 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	51.12s
✅ JSON	prettier	7	0	0	0	1.04s
✅ JSON	v8r	7		0	0	6.31s
⚠️ MARKDOWN	markdownlint	66	0	1	0	1.73s
✅ MARKDOWN	markdown-table-formatter	66	0	0	0	0.79s
✅ PYTHON	black	758	0	0	0	37.71s
✅ PYTHON	isort	758	0	0	0	2.73s
✅ REPOSITORY	checkov	yes		no	no	45.98s
✅ REPOSITORY	gitleaks	yes		no	no	17.47s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	20.64s
✅ REPOSITORY	syft	yes		no	no	4.06s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.21s
✅ REPOSITORY	trufflehog	yes		no	no	23.45s
✅ YAML	prettier	10	0	0	0	0.69s
✅ YAML	v8r	10		0	0	11.83s
✅ YAML	yamllint	10		0	0	0.52s

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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