Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• 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.
• 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".

A complete implementation of a proof-of-work blockchain in Python (~270 lines), covering:

• Block — a single block with index, timestamp, data, previous_hash, nonce, and hash. Supports equality comparison and dictionary serialization.
• calculate_hash() — computes the SHA-256 hash of a block's contents serialized as JSON (deterministic, no timestamp dependency).
• mine_block() — proof-of-work mining with adjustable difficulty (number of leading zeros required in the hash). Increments nonce until a valid hash is found.
• Blockchain — manages a chain of blocks with genesis block creation, block addition, chain integrity validation, and JSON export.

Wikipedia reference: https://en.wikipedia.org/wiki/Proof_of_work