feat(maths): add repunit theorem helpers (v3)

[nickzerjeski]

Describe your change:

Fixes #13999

• 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?

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".

[Copilot]

Pull request overview

Adds a small number-theory helper module for working with repunits (numbers of the form 111…1) and checking/deriving repunit divisibility properties, addressing the requested “repunit theorem” functionality in #13999.

Changes:

• Add has_repunit_multiple(divisor) to determine when a repunit multiple can exist (i.e., gcd(divisor, 10) == 1).
• Add least_repunit_length(divisor) to compute the smallest repunit length divisible by divisor using modular iteration (no large integer construction).

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[Copilot]

The module docstring introduces the repunit theorem but doesn’t include any reference URL. New algorithm modules in this repo typically include at least one Wikipedia (or similar) link; please add a reference (e.g., Repunit, multiplicative order, or the repunit divisibility theorem) to satisfy the project’s documentation expectations.

Suggested change

	there exists a repunit ``R_k`` divisible by ``n``.
	there exists a repunit ``R_k`` divisible by ``n``.
	References:
	- https://en.wikipedia.org/wiki/Repunit
	- https://en.wikipedia.org/wiki/Multiplicative_order