GeezerVison

Describe your change:

Adds an implementation of the Bailey-Borwein-Plouffe (BBP) formula in maths/bailey_borwein_plouffe.py.

The BBP formula is a spigot algorithm discovering in 1995 that allows direct extraction of any arbitrary hexadecimal (or binary) digit of $\pi$ without needing to calculate all preceding digits.

Key points:

• Implemented with pure standard library Python.
• Functions have complete type annotations and docstrings.
• Doctests cover standard and boundary cases and pass with pytest.
• Code conforms to formatting and lint rules (ruff).

References:

• https://en.wikipedia.org/wiki/Bailey%E2%80%93Borwein%E2%80%93Plouffe_formula
• https://www.experimentalmath.info/bbp-codes/bbp-alg.pdf
• Bailey, D. H., Borwein, P. B., & Plouffe, S. (1997). Mathematics of Computation.

Checklist:

• I have read the CONTRIBUTING.md guidelines.
• All functions have type hints and docstrings with doctests.
• All doctests pass locally.