Fix update link

[parweb]

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[parweb]

Re-checked this against current main (v20.1.2) before asking for anything — I think this PR is now obsolete and you may want to just close it.

The bug it was written for (stale Link references left in node.links after a link update, so removeLink would not clean the node side) no longer reproduces. createSingleLink now returns the existing link and mutates data in place, so the reference in node.links stays valid:

const g = createGraph();
g.addLink(1, 2, {w: 1});
g.addLink(1, 2, {w: 2});   // update
g.removeLink(g.getLink(1, 2));
// graph links: 0, node1.links.size: 0   ✅

Same result for multigraphs and self-loops.

What is left in the diff is the node.links Set -> Map change, which is a public API break — test/construction.js > add link adds link fails on it, since Array.from(node.links)[0] then yields [id, link] instead of the link. Not something worth landing to fix a bug that is already gone.

I have not touched the branch. Happy for you to close it — flagging rather than leaving it sitting in your queue.