[Feature] Resources: <mask> and non-rectangular (path) clips #216

Version: @tanstack/charts 1.1.0 (57774e1). Split from the comments on #128 and #191 so the ask stays open when those issues close.

Two resource types have no scene representation: a <mask> (first reported on 0.16.0, still absent in 1.1.0) and a clip that is not a rectangle.

Mask (from #128's comment)

An SVG <mask> with maskUnits="userSpaceOnUse" used as an edge fade on a live (rolling) line chart. It fades the line, its fill and the tip marker together, so a stroke gradient with opacity stops in gradients cannot replace it. We render it beside the chart svg and reference it by id, the same seam as patterns. packages/charts-core/src has no mask handling.

Path clip (from #191's comment)

• SceneGroup.clip is ChartBounds (x, y, width, height) (types.ts:1500).
• The SVG renderer always writes <clipPath><rect/></clipPath> (svg-renderer.ts:169-176), canvas uses context.rect(...) then clip() (canvas.ts:1243-1251), and hit testing intersects bounds (nearest.ts:198-203).
• The only path <clipPath> is private to the arc enter sweep (motion.ts:1653-1663). No scene node carries a path clip or mask.

Example: a highlight pulse that must be clipped to a non-rectangular silhouette (an isometric bar's front face, side face and lid). A rectangular clip lights the neighbouring faces.

Ask

Allow clip to be a rect or a path ({ path: string, fillRule?: 'nonzero' | 'evenodd' }), rendered as the <clipPath> child in SVG and a Path2D clip on canvas, with the path's bounds used for hit testing. For masks, a declared masks resource next to gradients (scoped by idPrefix), or the same generic resource slot that PR #166 adds for <pattern>.

Read from source, not run.

Related: #128 / PR #166 (<pattern> resources), #191 (gradients).