feat(csv): add generic row writing and cell update APIs
- Add CSV_MODE_READ_WRITE and CSV_MODE_WRITE_READ file open modes
- Implement csv_append_cell() to update individual CSV cells by row and column
- Add bounds and input validation for cell update operations
- Introduce MAX_COLS support for row parsing
- Add unit tests covering valid updates and error conditions
- Add csv_append_cell() demonstration in main.c
- Remove obsolete test data file