Implement XledgerExporter: CSV/JSON Payload Serialization

Define a `CsvJsonFileGenerator` utility class (or use it if already defined by the component) that accepts a list of maps and column order config, and produces both `csvBytes` and `jsonString`. The column order must be declared as a `static const List` constant in `XledgerExporter` (or its config) — never derived dynamically — so the order is auditable and matches the Xledger spec exactly. For the `XledgerExportPayload`, use a sealed payload type rather than putting both CSV and JSON in `dynamic payload` of `ExportPayloadWrapper` — this avoids casting at the call site. The BOM (byte order mark) for UTF-8 is optional in CSV; check Xledger's import tool requirements — some older tools require a BOM for proper UTF-8 detection.

If in doubt, include it (`0xEF, 0xBB, 0xBF` prepended to `csvBytes`). Do not use the `csv` pub package if it doesn't support all escaping requirements — a hand-rolled RFC 4180 serializer for this use case is 30 lines and far more predictable.

Unit tests using `flutter_test`. Required scenarios: (1) 3-record batch produces valid CSV with correct header and 3 data rows; (2) a string field containing a comma is properly quoted in CSV; (3) a string field containing a double-quote is escaped as `""`; (4) amount 150.00 appears as `150.00` not `150` or `1.5E2`; (5) JSON output has correct field names matching Xledger spec; (6) empty mapped records list returns `ValidationError`, not empty file; (7) `exportedClaimIds` in wrapper matches input claim IDs; (8) UTF-8 encoding verified by decoding `csvBytes` with `utf8.decode`. Snapshot test: generate CSV from a fixed 2-record input and assert exact string output matches a stored golden fixture.