Correlation-id registry for macula_content_transfer handles.

Each transfer mints a share_id (already published in sharing.put_started_v1 / sharing.get_started_v1 etc. by macula_feeder / macula_download). A caller that only saw the id in a published mesh fact — not the pid — needs a way to reach cancel/1,3; this table is that lookup. Monitor-based cleanup: an entry disappears the instant its owning process exits, no caller needs to unregister explicitly.

Table is protected — any process can read it directly via ets:lookup/2 (see whereis_share/1), only this gen_server ever writes to it.