Enh: search by Message-Id

It would be useful to allow search by Message-Id.

It's possible to use thread.lua, but that only works if there is only a single match

Start by reading thread.lua, which the issue identifies as the current way to search threads. Trace how it handles Message-Id matches and determine where search results are limited to one match. Done means Message-Id searches can return multiple matching messages or threads, with the behavior covered by the relevant existing search tests if available.

Written by the indexing model from the issue text.