Fix num_digits_fast for powers of ten

[fze-fze]

Describe your change:

Fix num_digits_fast() so it returns the correct digit count for powers of ten affected by floating-point logarithm rounding, such as 1000 and 10**15.

The function still uses math.log10() for the fast estimate, then corrects exact power-of-ten boundaries with an integer comparison. I also updated the related doctests so each function tests its own non-integer input path.

• 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".

Local verification

python3 -m doctest -v maths/number_of_digits.py
uvx ruff check maths/number_of_digits.py

Both checks passed locally.