Add move-to-front transform

[fze-fze]

Describe your change:

Add a pure Python implementation of the move-to-front transform in data_compression/move_to_front.py.

The new module includes:

• move_to_front_encode() to encode text as move-to-front indexes
• move_to_front_decode() to reconstruct text from encoded indexes
• alphabet validation for duplicate symbols
• doctests for normal use and invalid input
• a Wikipedia reference for the algorithm

This fits naturally beside data_compression/burrows_wheeler.py, whose module documentation already mentions move-to-front transform as a common follow-up compression step.

• 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 data_compression/move_to_front.py
uvx ruff check data_compression/move_to_front.py
uvx ruff format --check data_compression/move_to_front.py

I also ran a round-trip smoke test for several strings against a printable ASCII alphabet.