Function ensure_lane 

pub fn ensure_lane(
    repo_root: &Path,
    state: &Path,
    feature: &str,
    lane: &str,
    start: &str,
) -> Result<Worktree, Refusal>

A fresh lane worktree for lane of feature, started at start (the feature branch’s tip). A lane left behind is replaced: lanes are speccoder’s scratch space; their work reaches the feature branch only by merging.