Fix in_static_equilibrium for NumPy 2.x (2-D cross product removed) - #15110

Describe your change:

physics/in_static_equilibrium.py calls numpy.cross() on 2-D input vectors. NumPy deprecated 2-D cross products in 2.0 and removed them in newer releases, so on recent NumPy the doctest now raises:

ValueError: Both input arrays must be (arrays of) 3-dimensional vectors, but they are 2 and 2 dimensional instead.

Current master CI still passes only because its lockfile pins numpy==2.2.5; the failure surfaces on newer NumPy (e.g. the free-threaded 3.14t job resolves numpy==2.5.2). This is a latent break that will hit master on the next NumPy bump.

The 2-D "cross product" is just the scalar z-component x·Fy − y·Fx per row, so I compute it directly. Behaviour is identical to the old cross() and works across all NumPy versions. Doctests and the __main__ self-checks pass unchanged.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? — no, existing doctests unchanged

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one issue, please mention the issue number.