sanitize
Coerce raw into a filesystem-safe form suitable for both a directory name and a database file stem. Replaces any character outside [A-Za-z0-9_-] with _, trims the result to at most 64 characters, and returns "Untitled" when the coerced form is empty (for example, when the user typed only whitespace).
Stable and idempotent: a value that already satisfies the rules is returned unchanged.