Optimize ASCII lowercase conversion performance

[Miladkhoshdel]

Describe your change:

Improve the performance of the existing ASCII lowercase conversion by replacing the generator expression with an explicit loop.

The updated implementation:

• Computes each character's ASCII value only once.
• Uses module-level constants for the ASCII uppercase range and case offset to improve readability and avoid magic numbers.
• Builds the result using a list and joins it at the end.
• Preserves the existing behavior and doctests.

Benchmark using:

python -m timeit \
  -s "from strings.lower import lower; s = 'Hello WORLD 123!' * 1000" \
  "lower(s)"

Results on my machine:

• Previous implementation: ~1.08 ms per loop
• Updated implementation: ~772 µs per loop
• Runtime reduction: ~28.5%

• 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".

[cclauss]

Please add a benchmark that proves that the proposed changes provide significant runtime improvements.