Lazily add edges

When an edge is added, non-existing nodes are added automatically.

However, sometimes the edges of a node are added while the other nodes, including those to be connected, haven't been added yet. Therefore it would be very helpful if those edges were added without creating non-existing nodes.
Even better: An event is emitted when those yet non-existing nodes are queries, so they can be dynamically added (e.g. by network request).

Start by tracing the existing edge-addition API and its tests to understand how missing nodes are currently created. Clarify whether deferred edges should remain pending and how an event would expose queried nodes. Done means edges can be added without creating missing nodes, with a defined mechanism for dynamically supplying those nodes.

Written by the indexing model from the issue text.