feat: Excel table option, context param, streaming addObject

[productdevbook]

Summary

• writeObjects table option — auto-generates native Excel table (ListObject) from columns with totals, style, autoFilter
• writeObjects context param — injects runtime context into column callbacks via item.__ctx
• XlsxStreamWriter.addObject() — write objects using ColumnDef value accessors (key, dot-path, function)

API Examples

Excel Table (#142)

const xlsx = await writeObjects(data, {
  columns: [
    { key: 'product', header: 'Product' },
    { key: 'revenue', header: 'Revenue', numFmt: '$#,##0' },
  ],
  table: {
    name: 'SalesTable',
    style: 'TableStyleMedium2',
    showTotalRow: true,
    totals: { revenue: 'sum' },
  },
})

Context (#149)

const xlsx = await writeObjects(data, {
  columns: [{
    header: 'Converted',
    value: (item) => item.price * item.__ctx.rate,
  }],
  context: { rate: 0.92 },
})

Streaming addObject (#150)

const writer = new XlsxStreamWriter({
  name: 'Data',
  columns: [
    { value: 'user.name', header: 'Name' },
    { key: 'score', header: 'Score' },
  ],
})
writer.addObject({ user: { name: 'Alice' }, score: 95 })
const xlsx = await writer.finish()

Test plan

• 2087 tests pass (7 new)
• TypeScript typecheck clean
• Lint + format clean

Closes #142, #149, #150

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