DATEV exporter 0.2.0 API

Javadoc for the five Java modules published to Maven Central.

datev-exporter-core — Core schema, metadata, CSV codec and validation contracts
datev-exporter-plain — Fixed v13/v12 complete-file and forward-only streaming exporters
datev-exporter-field-validator — Optional semantic field validation
datev-exporter-advanced — Custom-header exporter with built-in validation modes
datev-exporter-advanced-univocity — Optional Univocity CsvWriter interoperability