Enhanced NXT class-year cutoff

For GiveCampus constituent data and imports.

For the enhanced NXT class-year import, a two-digit year below the cutoff receives the century 2000; a value at or above it receives 1900. At cutoff 50, 30 becomes 2030. Cutoff 30 maps 30–99 to 1930–1999 and 00–29 to 2000–2029. No cutoff can distinguish 1930 from 2030 if both are supplied as 30. Prefer four-digit source years where available: these pass through unchanged. Test representative rows and verify the actual import configuration before changing the school-wide cutoff.