The forge half of zit#144. A stack is a chain of dependent landings that land bottom-up; quarry renders the chain, refuses accept above the bottom, and flags a segment whose base was rejected.

No new index and no new artifact: the edge is already implied by a landing's target, so a stack is a reading of landings the way a landing is a reading of forum posts. closed check-in: 041ca46258 user: dmestas tags: strata-rfc