When clearing an MC6 preset, emulate the "clear" message sent from the web UI

I think the clear implementation we have is _only_ focused on PC messages. Over time we'll want to have the thing fully reset so that other functionality goes away as well.

Start by locating the existing MC6 preset-clearing implementation and the code that emits PC messages, then compare its behavior with the web UI's “clear” message. Done means clearing a preset emulates that message and resets the other functionality described in the issue; add or update tests if this path has existing coverage.

Written by the indexing model from the issue text.