Item to define custom non-ProjectReference edge in graph

Several build processes (repos, SDKs, and NuGet packages) have a pattern where they want to do a thing "once per build"--maybe set up versioning, or run some code generator, or something.

Ideally it would be easy to express this in a `-graph -isolate` compatible way by creating an item something like

```xml

```

This would be a totally new edge, ignoring target propagation.

Start by tracing the existing -graph -isolate handling and how graph edges propagate targets. Compare the proposed MSBuildGraphEdge metadata (Include, Targets, Properties) with current edge items; done means a custom edge can run once per build without target propagation and remains compatible with isolation.

Written by the indexing model from the issue text.