Add Boyer-Moore-Horspool string-search algorithm

[ChrisJr404]

Describe your change

Adds the Boyer-Moore-Horspool string-search algorithm under strings/boyer_moore_horspool.py.

Horspool (1980) is the most widely-taught simplification of the Boyer-Moore family: it keeps only the bad-character shift table and uses the rightmost character of the current text window to drive the shift. It is sub-linear on average (~O(n / m) on random text), worst-case O(n * m), and uses O(sigma) memory where sigma is the size of the alphabet that appears in the pattern. It is often the simplest sub-linear matcher to implement and is the algorithm used internally by libraries such as glibc's memmem for short patterns.

The repository already contains the original Boyer-Moore (strings/boyer_moore_search.py) and Knuth-Morris-Pratt (strings/knuth_morris_pratt.py); Horspool is a distinct, well-known variant and was missing. I confirmed the file does not already exist (rg horspool only matches the dictionary corpus) and DIRECTORY.md had no entry for it.

Reference: https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore%E2%80%93Horspool_algorithm

The module exposes:

• boyer_moore_horspool_search(text, pattern) -> int - first-match index, -1 on no match, behaves like str.find for the empty-pattern edge case.
• boyer_moore_horspool_search_all(text, pattern) -> list[int] - all (overlapping) match indices.
• _build_shift_table(pattern) - private helper, also doctested.

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. The file does not require a third-party library.
• 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".

Tests

• 18 doctests across the three functions, covering empty pattern, empty text, pattern longer than text, no match, single match, overlapping matches, and parity with str.find.
• python -m doctest strings/boyer_moore_horspool.py -v -> 18 passed.
• python -m pytest strings/boyer_moore_horspool.py --doctest-modules -> 3 passed.
• Local fuzz of 2000 random inputs over a 4-letter alphabet against str.find and a naive all-occurrences oracle: 100% agreement.
• pre-commit run --files strings/boyer_moore_horspool.py DIRECTORY.md passes (ruff check, ruff format, codespell, auto-walrus, validate-filenames).

DIRECTORY.md updated alphabetically in the Strings section.