users.csv userIds field is never populated

The `userIds` column in users.csv is declared in UserFile.cs but never mapped in Map(), resulting in always-empty output.

The OneRoster 1.1 Best Practice Guide (section 3.1.3) recommends populating this field with machine-readable identifiers (e.g. LTI, LDAP) for bridging purposes.

**File to fix:** Models/Exports/UserFile.cs -- populate UserIds in Map() with a meaningful identifier (e.g. the username or a formatted ID).

**Impact:** External systems relying on the userIds field for identity bridging will not find data.