Allow Outputting Anchors for Tree Nodes

[johannes-wolf]

Discussed in #698

^{Originally posted by noahjutz September 23, 2024}
Is it possible to refer to a tree's node's position from outside of a tree?

When I try

cetz.tree.tree(
  // ...
  draw-node: (node, p) => {
    if node.content.text == "1" {circle((), radius: 0, name: "anchor_1")}
    else if node.content.text == "2" {circle((), radius: 0, name: "anchor_2")}
  }
)

line("anchor_1", "anchor_2")

I get Assertion failed: Unknown element 'anchor_1' in elements ().

Motivation: I want to draw arrows between sibling nodes in a tree.

[johannes-wolf]

Forward anchors from the draw-node function to the tree group.