AttachStrata reads the chain from target -> branch across landings: base, position, depth, and why a chain cannot be walked — a cycle, or a rejected base. Derived at read time, never stored, so a segment retargeted by zit#144 after its base lands simply stops being one.

landing.list carries the reading and counts DECISIONS: a stack of five is one, the base's, because nothing above it can be acted on. landing.get carries the chain for the record's map and reports a rung above the bottom as stacked, which the merge probe must not overwrite: its answer is true and useless there. check-in: 3e3ad17751 user: dmestas tags: strata-ui