Support NumPy 2.x, including NumPy 2.4

[taylorbell57]

Remove the outdated numpy<2 dependency restriction and fix compatibility issues in spot occultation and plotting.

• Replace the removed ndarray.ptp() method with np.ptp().
• Extract scalar spot coordinates explicitly, avoiding array-to-scalar conversion rejected by NumPy 2.4.
• Correct fast/slow test parameters to pass actual booleans; previously, both string values exercised fast mode.
• Preserve the 350 ppm fast-mode STSP tolerance and document a 500 ppm bound for the existing slow-mode approximation. Its 443 ppm residual is identical in upstream main and this PR.
• Add regression tests for inclined spot crossings in both modes and for plotting.

Validation: all 10 core tests pass locally with NumPy 1.26.4 and 2.4.6. CI passes on Python 3.10 and 3.11, including the JAX test job and code style checks. The Read the Docs build also passes.

Closes #29.