Connection edges drawn over ports make the ports hard to select

From mbeddr.core created by [wsafonov](https://github.com/wsafonov) : mbeddr/mbeddr.core#1507

When a port is used to connect nodes both from the parent diagram and from the subdiagram, the connection edge could be drawn over the port and make the port hard to select at that place, because the edge would be selected instead. Either the lines should get less priority during selection, or the connection point of the edge to the port should be flexible to better support scenarios with connections from opposite sides.

Start by reproducing the parent-diagram and subdiagram connection scenario described in the issue, using the linked screenshot as context. No implementation file or test is named; the work is complete when the port remains selectable where an overlapping connection edge meets it, without breaking edge selection.

Written by the indexing model from the issue text.