Function normalize_root_input 

pub fn normalize_root_input(raw: &str) -> Result<PathBuf>

The full epicenter setup orchestration: compute the decision, make the directory, and write/merge the cargo config. Normalize a user-provided --cache-root value.

• cargo template strings ({…}) pass through untouched — they resolve at build time via cargo, exactly like the default template;
• ~ expands to the home directory;
• relative paths resolve against the current directory;
• absolute paths stay as-is.