[fix] useResponderEvents: negotiate from a common ancestor with no responder ancestors

[Pduhard]

Fix #2916

getLowestCommonAncestor read the first pair twice, so it never compared the last one. When that was the only common ancestor, it returned null and no negotiation happened. It now advances the index before reading.

Test: Negotiation › negotiate from first common ancestor when it has no responder ancestors. It fails without the fix. Its neighbour, negotiate from first common ancestor when there are siblings, passes only because a grandParent is connected to the responder system.

A second touch starting on the sibling goes through the same path. The test uses a mouse move, as in the issue.

format, flow, lint and unit pass. The fix also applies as is to 0.22-dev.

Written with Claude Code; I reviewed the code and the tests.

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