feat(excel): add toJson/fromJson &generateXFile; fix column/row sizing and cell parsing

[mkidv]

Summary

Adds a JSON API, an XFile export helper, and fixes several parsing issues (column/row sizing and cell types). Strengthens cell parsing (formulas/inline strings/num formats) and extends the test suite.

Motivation

• Convert workbooks to/from a JSON view for storage, diffing, or APIs.
• Provide a first-class way to get an XFile (Flutter/web/desktop) without manual plumbing.
• Address sizing inconsistencies and parsing edge cases across Excel / Google Sheets / LibreOffice.

What’s changed

• Added
  • Excel.toJson() / Excel.fromJson(Map) and toString() ≡ jsonEncode(toJson()).
  • Excel.generateXFile({fileName}) → returns an XFile with the correct MIME & name.
  • Browser saving helper now also exposes generateXFile.
• Fixed
  • Column widths & row heights parsing: defaults + per-col/per-row (<sheetFormatPr>, <col width>, <row ht>).
  • Cell parsing robustness:
    • Formula-first handling (<f> preferred over <v> when present).
    • Safe sharedStrings lookup (bounds checks).
    • Full t="inlineStr" support.
    • Bounds checks for numFmtId/style indices with sensible fallbacks.

API snippets

// JSON round-trip
final json = excel.toJson();
final excel2 = Excel.fromJson(json);

// XFile generation (Flutter/web/desktop)
final xf = excel.generateXFile(fileName: 'report.xlsx');
// -> XFile(name: report.xlsx, mime: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet)

Notes on JSON format

• Dates/DateTimes use ISO-8601 UTC strings with milliseconds:
  • DateCellValue → YYYY-MM-DDT00:00:00.000Z
  • DateTimeCellValue → YYYY-MM-DDTHH:mm:ss.SSSZ
  • Times are HH:mm:ss[.SSS].
• Formulas are strings starting with =….
• Numeric/bool types stay typed on re-import; text remains text.

Tests

• JSON view stable across encode/decode.
• inlineStr cells read correctly.
• XFile generation returns correct name + MIME.

Backward compatibility

• No breaking changes