Accept integer scalar cubic_call inputs

[mcdigman]

Summary

• Accept Python and NumPy integer scalar xint inputs in cubic_call and AkimaSpline.__call__ at runtime, but not for type checking
• Extend the Numba overload to dispatch integer scalar inputs through cubic_call_scalar.
• Replace integer-rejection assertions with int-vs-float equivalence coverage for Python and njit callers.

Tests

• pytest -q
• ruff check pyakima/pyakima.py tests/test_pyakima.py
• ruff format --check pyakima/pyakima.py tests/test_pyakima.py
• mypy --ignore-missing-imports --explicit-package-bases pyakima tests/test_pyakima.py