feat(legend): support pager.hugContent to hug and center paged legend content

[g1f9]

🤔 This is a ...

• New feature

🔗 Related issue link

N/A

🐞 Bugserver case id

N/A

💡 Background and solution

Problem. When a discrete legend paginates, the pager is pinned to the far end of the available legend space (_updatePositionOfPager: x = compWidth - pagerWidth for horizontal layouts), while the items stay at the start. The paged legend therefore always renders justified — items on the left, pager on the right — and its AABB always occupies the full available width, because the paging clip viewport (clipGroup in _renderPager) is created with the fixed size contentWidth = maxWidth - pagerWidth - pagerSpace and clip: true, which pins the component bounds regardless of the actual content extent.

This makes it impossible for outer layouts to center "items + pager" as one block — a very common legend design (the whole paged legend centered under the chart). For example, VChart's legend position: 'middle' centering (offsetX = (rectWidth - boundsWidth) / 2) becomes a no-op for paged legends since boundsWidth always equals the full available width.

Solution. Add an opt-in pager.hugContent option (default false, fully backward compatible). When enabled:

1. the pager is placed right after the last legend item, with pager.space as the gap; when the content fills the available space it falls back to the original edge-pinned position (Math.min clamp);
2. the paging clip viewport shrinks to the actual content extent (Math.min(contentWidth, itemsContainer.AABBBounds.width()), and the height counterpart for vertical layouts). Items wrap before contentWidth, so shrinking the viewport never clips content.

With both applied, the legend bounds hug "items + pager", and outer layouts can center the whole block.

Usage:

const legend = new DiscreteLegend({
  layout: 'horizontal',
  maxWidth: 400,
  maxRow: 1,
  items,
  pager: { hugContent: true }
});

Before (default, unchanged): [item1 item2 < 1/3 >]
After (hugContent: true), with an outer centering layout: [ item1 item2 < 1/3 > ]

📝 Changelog

Language	Changelog
🇺🇸 English	feat(legend): add pager.hugContent for discrete legend — place the pager right after the last item and shrink the paging viewport to the content extent, so the paged legend bounds hug "items + pager" and outer layouts can center the whole block. Defaults to false (no behavior change).
🇨🇳 Chinese	feat(legend): 离散图例新增 pager.hugContent 配置——翻页器紧跟最后一个图例项排布，同时分页视口收缩到实际内容尺寸，使「图例项 + 翻页器」的包围盒随内容自适应，便于外部布局整块居中。默认 false，不改变现有行为。

☑️ Self-Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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