Extreme low-power mode on NRF5340

This is not so much an issues as a discussion, before we start implementation, since I couldn't find a forum for this.

We would like to completely disable Nimble and the net-core on the NRF5340 for long periods. When needed, the Nimble
stack (and Net-core) should be restarted and used for communications. Nimble itself doesn't appear to have deinit() functions (yet).

What we plan to do is:
- Net-core: wait for Radio to be idle and then disable the Radio (and all other used peripherals). Then signal app-core
- App-core: wait until net-core is done with shutdown and then really shut it down.
- App-core Nimble stack: stop tasks, stop timer, remove queues (or clear), reset some static global structs to zero.

The idea is that we dont have to modify the init steps this way, so future merges remain easy.

Am I missing something with this approach?

The issue provides no files, tests, or entry points; begin by tracing Nimble initialization, task and timer ownership, and the NRF5340 net-core/app-core shutdown flow. Review how Radio and other peripherals are stopped and whether Nimble exposes lifecycle support; the issue does not define a decided implementation or completion criteria.

Written by the indexing model from the issue text.