Function fallback 

pub fn fallback(kind: BranchKind, issue_number: u64, title: &str) -> BranchName

Builds the deterministic fallback branch name for an issue: kind, the issue number, and the title reduced to a lowercase kebab-case slug via crate::slug::slugify ("issue" when the title yields nothing usable). The result always satisfies parse_and_validate, so worktree creation can proceed no matter what a generation step produced (issue #98).