Add Hirschberg algorithm for longest common subsequence

[KHG420]

Describe your change

• 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 Hirschberg's algorithm for reconstructing a longest common subsequence with linear-size dynamic programming rows. The repository already has a full-table LCS implementation; this adds the divide-and-conquer approach with a different space complexity. Reference: https://en.wikipedia.org/wiki/Hirschberg%27s_algorithm

Validation:

• python3 -m doctest -v dynamic_programming/hirschberg_algorithm.py (9 passed)
• uvx --from pytest pytest dynamic_programming/hirschberg_algorithm.py --doctest-modules -q (3 passed)
• uvx ruff check dynamic_programming/hirschberg_algorithm.py (passed)
• uvx ruff format --check dynamic_programming/hirschberg_algorithm.py (passed)
• Compared LCS length and subsequence validity with the existing implementation for 3,969 string pairs.

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".