Description

Fixes the probability mutation defaulting logic so mutation_prob=0 is respected instead of being treated as omitted.

Previously, Chromosome.mutate() used a truthiness check, so passing 0 fell back to the default 1 / len(self.data) probability and could still mutate genes. The new check defaults only when mutation_prob is None.

Adds a regression test covering the zero-probability case.

Related issue

Closes #14

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Documentation only

Checklist

• I have signed the CLA (the CLA Assistant bot will prompt external contributors)
• My commit messages follow Conventional Commits
• ruff check . and black --check . pass
• mypy genetic_algorithm passes
• pytest passes
• The engine remains dependency-free (standard library only)
• I have added/updated tests where relevant
• I have updated documentation where relevant
• No secrets, API keys, internal URLs, or proprietary content are included