thread_shares (row 193 design 5.2) with its index and append-only triggers; the
3l-3m step only adds them. thread.share, thread.unshare and thread.shares for a
thread's owner; #readable holds design 5.4's four rules, #threadRead serves
thread.messages only, and thread.list {shared:true} adds owner. replies and
trail show shared user threads to the owner and readers. COUNTED, LISTED,
timedTables and the clock list take the table. Tests: thread-shares.test.mjs
and the 3l→3m pins. Packet agents/launchpad/work/queue-206/BUILD.md. Not run
yet: node waits for Sage's CLEAR (sage-2789).