Function compute_from_spec_dir 

pub fn compute_from_spec_dir(specs_root: &Path) -> Result<PopularityMap>

Compute a popularity map from all tool spec directories under specs_root.

Walks specs_root/**/*.toml, parses each as a BuildSpec, and counts how many specs declare each package name. The resulting map is sorted deterministically (by name) inside save() via serde_json’s map ordering.