feat(xlsx): @gridsheet/xlsx — xlsx ⇄ GridSheet converter

[righ]

What

New headless package @gridsheet/xlsx that converts between .xlsx and GridSheet — a TS-native OOXML reader/writer scoped to what CellType represents, using fflate for zip. It does not wrap exceljs/SheetJS (both effectively unmaintained); exceljs was used only as a reference for the OOXML shapes.

API

import { fromXlsx, toXlsx } from '@gridsheet/xlsx';
import { buildInitialCells } from '@gridsheet/react-core';

const parsed = fromXlsx(bytes);              // { [sheet]: { matrices, cells } }
<GridSheet initialCells={buildInitialCells(parsed.Sheet1)} />

const out = toXlsx({ Sheet1: sheetRef.current });  // Uint8Array

Supported (both directions)

• Values (string / number / boolean) and formulas as their =... text, including cross-sheet references (=SUM(Sales!D3:D5), quoted names like 'Meta Data'!B3).
• Multiple sheets, in workbook order.
• Cell styles ↔ GridSheet style: background, text color, bold, italic, underline, and horizontal/vertical alignment (justifyContent/alignItems).
• Column widths / row heights ↔ header cells (ch(col)/rh(row)), px↔xlsx units (sizes written when the input is a live sheet).
• Dates import as their raw Excel serial (no number-format support yet); exported values write as ISO strings.

Unsupported features (merged cells, number formats, borders, charts/images) are ignored gracefully on import — a styled, merged, multi-sheet workbook reads fine.

Tests / verification

• Jest (30): A1 helpers, a minimal XML parser, toXlsx↔fromXlsx round trips (types, formulas, escaping, shared-string dedup, sparse), a hand-built workbook exercising inlineStr / cached formulas, a complex openpyxl-written fixture (styles, merges, sizes, cross-sheet), and engine-integration specs (cross-sheet re-evaluation, style/size round trip).
• Interop verified both ways against openpyxl (an independent OOXML implementation): our output opens cleanly (fill/font/alignment intact) and we read openpyxl's output correctly.
• Storybook IO/Xlsx demo (ships a complex sample.xlsx; import a file / download) + Playwright e2e (e2e/xlsx.spec.ts) asserting imported styles render and cross-sheet formulas re-evaluate live.

Wiring

Root scripts (build:xlsx / jest:xlsx / typecheck:xlsx and the :all aggregates), a dependency-ordered upload-xlsx publish job in release.yaml, and AGENTS.md package table / testing policy.

Notes

• npm publish prerequisite: @gridsheet/xlsx must be registered as an npm OIDC trusted publisher before the upload-xlsx job can publish it (the job is || true, so it won't fail the release until then).
• This branch also carried the fixed-height sheet-resize fix, which already shipped in 3.4.4; it is therefore absent from this diff (already on master).

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