First release. A native .xlsx writer for Dart, binding libxlsxwriter 1.2.2 over FFI, with libxlsxwriter and zlib 1.3.1 vendored and compiled from source at build time (no system dependencies on macOS, Linux, or Windows).

• Workbook and Workbook.constantMemory for in-memory and constant-memory writing, with a NativeFinalizer so a forgotten workbook is freed.
• Worksheet writes: writeString, writeNumber, writeBool, writeFormula, writeDateTime, writeUrl, writeBlank.
• Layout: setColumn, setColumnWidth, setRow, mergeRange, freezePanes.
• Format with bold, italic, underline, fontName, fontSize, fontColor, backgroundColor, numberFormat, align, verticalAlign, textWrap, border, and borderColor.
• libxlsxwriter error codes surfaced as XlsxWriterException with the human-readable message.