[codex] Add backend isscalar function

[FlorianPfaff]

Summary

• add isscalar to the shared backend interface
• implement isscalar for the NumPy, PyTorch, and JAX backends, with the autograd mirror updated as well
• add a backend smoke test that verifies NumPy-style scalar semantics across the backend CI matrix

Why

Some code paths need a backend-level scalar check instead of reaching for backend-specific helpers directly. This change exposes isscalar consistently through pyrecest.backend.

Impact

Backend consumers can now call pyrecest.backend.isscalar(...) regardless of whether the active backend is NumPy, PyTorch, or JAX.

Validation

• PYRECEST_BACKEND=numpy python -m pytest -p no:cacheprovider --rootdir . -q pyrecest/tests/test_backend_interface.py
• PYRECEST_BACKEND=pytorch python -m pytest -p no:cacheprovider --rootdir . -q pyrecest/tests/test_backend_interface.py
• PYRECEST_BACKEND=jax python -m pytest -p no:cacheprovider --rootdir . -q pyrecest/tests/test_backend_interface.py

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ JSON	prettier	2	0	0	0	0.44s
✅ JSON	v8r	2		0	0	2.45s
✅ MARKDOWN	markdownlint	2	0	0	0	0.64s
✅ MARKDOWN	markdown-table-formatter	2	0	0	0	0.19s
✅ PYTHON	bandit	353		0	0	5.17s
✅ PYTHON	black	353	8	0	0	8.2s
✅ PYTHON	flake8	353		0	0	2.92s
✅ PYTHON	isort	353	13	0	0	0.69s
✅ PYTHON	mypy	353		0	0	4.84s
✅ PYTHON	pylint	353		0	0	102.64s
✅ PYTHON	ruff	353	13	0	0	0.07s
✅ REPOSITORY	checkov	yes		no	no	21.23s
✅ REPOSITORY	gitleaks	yes		no	no	6.07s
✅ REPOSITORY	git_diff	yes		no	no	0.08s
✅ REPOSITORY	secretlint	yes		no	no	7.37s
✅ REPOSITORY	syft	yes		no	no	5.86s
✅ REPOSITORY	trivy-sbom	yes		no	no	2.14s
✅ REPOSITORY	trufflehog	yes		no	no	18.61s
✅ YAML	prettier	4	0	0	0	0.46s
✅ YAML	v8r	4		0	0	4.55s
✅ YAML	yamllint	4		0	0	0.4s

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