The vocabulary of propagation: when changed keys of a child feed a parent, which of the parent's keys become dirty?

• {:keys, mapped} — a bounded, per-key fan-out (identity, or a remap).
• :all — the change fans out to the WHOLE parent cell. Needed when one input change can affect arbitrary output keys: a fold's aggregate, or a vanished key a relation must re-judge.

Getting that distinction right is what keeps a cascade O(real changes). Escalating to :all is always CORRECT and sometimes wasteful; passing keys through when the grain actually changed is neither — it strands rows that needed repricing, which reads as data that quietly went stale.

ReactiveDag.Node.KeyRule decides it from what the node declared (key_rule: :identity | :all, or a recompute_by unit), and the drain always uses that one. This module holds the shared types plus the trivial identity rule, which is the answer for a same-grain map and the fallback for a hand-assembled cell that declares nothing.