SankeyKit 1.0.0

First public release.

Added

• Marks and a result builder. SankeyChart takes SankeyLink and SankeyNode marks through
  SankeyContentBuilder, where if, if/else and for work exactly as they do in a
  ViewBuilder.
• A data-driven initializer. SankeyChart(data) { … } builds the same chart from a collection,
  one group of marks per element.
• Labelled values. SankeyValue mirrors PlottableValue; its labels become the descriptions
  VoiceOver reads.
• Implicit nodes and inferred columns. Naming a node in a link creates it; columns follow from
  longest-path layering, with per-node overrides. A cyclic graph names the cycle instead of
  crashing.
• Chart modifiers. sankeyNodeWidth(_:), sankeyNodeSpacing(_:), sankeyNodeCornerRadius(_:),
  sankeyLinkSpacing(_:), sankeyLinkCurvature(_:), sankeyLinkOpacity(_:) and
  sankeyColorScale(_:) — environment-backed and composable, like chartXAxis and friends.
• Selection. Bind a SankeySelection? with sankeySelection(_:); a tap or an accessibility
  activation lights one flow while the rest dims.
• Animation. Changing a value inside withAnimation morphs the ribbons rather than jumping
  them: each ribbon interpolates through its endpoints and rebuilds its curve every frame.
• Accessibility. Every node and every ribbon is its own element, read in column order, with
  labels, values and selection traits.
• A pure layout engine. SankeyGraph and SankeyLayout are view-free value-type computations
  covered by 112 unit tests.