maths: add Keith number algorithm

[shad0w0P]

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?
  Add Keith number (also known as a repfigit number) detection and search under maths/.

• Implement is_keith_number(number: int) -> bool using an $O(1)$ rolling sum sliding window approach.
• Implement find_keith_numbers(limit: int) -> list[int] to find all Keith numbers up to a specified limit.
• Include Wikipedia and OEIS reference links.
• Include 28 comprehensive doctests covering known numbers, non-Keith numbers, edge cases, and type errors.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• 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 or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".