feat: support function type for discrete legend maxRow / maxCol

[g1f9]

🔎 Background

The discrete legend's maxRow / maxCol currently only accept a fixed number, decided before layout. There is no way to let the row / column count depend on the space actually allocated to the legend at layout time (e.g. "allow more rows when the legend area is tall and narrow"). A static, layout-blind number means the legend can paginate and hide items even when the available space could fit them — and it can't adapt on resize.

✨ What's changed

maxRow / maxCol of the discrete legend now also accept a callback:

type DiscreteLegendArrangeCount =
  | number
  | ((ctx: { rect: ILayoutRect; orient?: IOrientType; id?: StringOrNumber }) => number);

The callback is evaluated inside getLegendAttributes during layout, against the legend's allocated rect, so the row / column count can adapt to the available space. Since it runs on every layout pass, it also re-evaluates on resize automatically.

Fully backward compatible — numeric maxRow / maxCol pass through unchanged.

🌈 Usage

legends: {
  type: 'discrete',
  orient: 'bottom',
  // allow as many rows as roughly fit 25% of the legend's available height
  maxRow: ({ rect }) => Math.max(1, Math.floor((rect.height * 0.25) / 26))
}

✅ Test

Added packages/vchart/__tests__/unit/component/legend/discrete-legend.test.ts:

• function maxRow is evaluated against the layout rect;
• function maxCol receives { rect, orient, id } in its context;
• numeric maxRow / maxCol keep working unchanged.