v0.22.0

Flow builder: host-shaped documents

• box-flow-builder reads documents through a model (children, title,
  description, status, addBranchLabel), so hosts pass their own node
  objects in any shape; FlowBuilder<N> is generic over the node type and the
  default model keeps reading body / branches.
• Branch management: add and remove controls emit branch-add-request and
  branch-remove-request for the host to apply.
• start-label / end-label replace "Flow starts" / "Flow ends".
• FlowKind.tone (accent | neutral) colors icons; the chooser shows icons.
• setValidation(…, { showMessage: false }) marks a step without the builder's
  own error line; refreshNode(node) re-renders one card and the inspector heading.
• Card, spine and chooser parts are exported through box-flow-builder.