Fixes#79
Implement the date segment type in the part number generator. Uses Go's
time.Format with the segment's Value field as the layout string.
- Default format: 20060102 (YYYYMMDD) when no Value is specified
- Custom formats via Value field: "0601" (YYMM), "2006" (YYYY), etc.
- Always uses UTC time
- Add 3 tests: default format, custom YYMM format, year-only format