Allow breaking down tree explorer by process/thread and other arbitrary types

We should add ways to add arbitrary top-level and bottom-level breakdowns to the tree explorer.

For example, breaking down roots by a Java heap type *or* breaking down by the process or thread or cpu or core type etc.

There's lots of uses for this.

Start by locating the tree explorer entry point and the code that currently defines its root and bottom-level breakdowns. Review how process, thread, CPU/core, and Java heap types are represented, then clarify the intended extension points and supported combinations with maintainers. Done means arbitrary top-level and bottom-level breakdowns work in the explorer with coverage for the requested examples.

Written by the indexing model from the issue text.