A key channel

I think for interaction (and perhaps even for tooltips) we’ll want a concept of a key channel: an ordinal channel that defines the identity of each instance. This way we can unambiguously say whether a given instance (a given data point) should appear selected/highlighted during interaction, similar to how the fx and fy channels unambiguously say in which facet the instance should appear.

We can have some default too, of course, say based on the strict identity of the data and the zero-based index. But we’ll want this concept to better support interaction with layered marks.

(And then the next thing we want is a way to set the selection interactively, to override mark options when an instance is selected, and to re-render marks incrementally when the selection changes.)

The issue names no files, tests, or entry points. Start by locating the channel and interaction implementations, then clarify the scope and acceptance criteria for a key channel, layered marks, and the proposed selection behavior before attempting work.