CsvReader / XlsxReader / FieldParser - zero-dep but single-consumer (plan G)

[andregoepel]

finance-app Connectors contains three genuinely reusable, dependency-free readers: CsvReader.cs (117 lines, RFC 4180), XlsxReader.cs (139 lines, minimal SpreadsheetML), FieldParser.cs (88 lines, culture-aware date/decimal parsing).

All fail the '>1 repo' intake criterion today. Revisit when a second repo needs file parsing.