Feature Request: Configurable retention policy for serial console output buffer of an instance

It would be useful to retain and access the scrollback buffer of an instance's serial console at arbitrary times, regardless of an instance's state (e.g. if it goes into Stopped or Failed deliberately or otherwise).
This applies both for omicron development (e.g. debugging a guest crash while making changes to propolis) and for users who simply encounter a bug in their own guest OS / application (e.g. reviewing a kernel panic written only to the tty). Even outside of these circumstances, it can sometimes be useful to retain logs as part of a security policy or simply for historical/informational purposes.

Feature Request:

• Allow a user to decide whether the buffer should be cleared upon an instance stop vs an instance delete, and to choose a max size on the buffer (presumably in lines or bytes).
• Allow the buffer to be retrieved regardless of instance state.

No files or tests are named. Start by tracing the existing serial-console buffer behavior and the instance stop and delete lifecycle paths, then identify the API surfaces for configuring retention and retrieving output; done means retention choices and retrieval work in every requested instance state.