Add --version/-V CLI flag

[dyson-025]

Closes #38

Summary

• Added support for --version and -V as eager CLI options.
• Preserved the existing version subcommand.
• Added a test to verify that --version prints the installed package version and exits successfully.

Testing

• Ran pytest.
• Ran ruff check ..
• Verified modelfuzz --version, modelfuzz -V, and modelfuzz version.

[higagan]

Verified locally — works exactly as intended.

• modelfuzz --version and -V both print the version and exit 0; the existing version subcommand still works; a bare modelfuzz still shows usage.
• is_eager=True on the callback option is the right call — the flag resolves before a subcommand is required.
• ruff clean, 97 tests pass (including your new one).

Two tiny things for next time, neither blocking: the test could assert -V as well as --version, and the local from modelfuzz import __version__ inside the test is redundant since the module already imports it. Merging as-is.