Function calculate_next_version 

pub fn calculate_next_version(
    current_version: Option<Version>,
    classified_commits: &[(CommitSummary, BumpLevel)],
) -> Version

Calculate the next version based on commits since last tag.

Algorithm (SRS §5.3):

1. Get current version from last tag
2. Classify commits to find highest bump
3. Increment version using highest bump
4. Handle edge cases: no tag → use initial version (0.1.0)