A GENERIC ReactiveDag.KeyRule for graphs declared with ReactiveDag.Node.
Node records each node's key_rule (:identity | :all) in cell.meta;
this reads it:
:all— any input change escalates to a whole-cell recompute (the drain turns this into"*"). For aggregate / cross-range (reduce) cells.:identity— a changed input key maps to the same output key (pass through). For key-local (map) cells. The default.
This is the uniform per-cell rule (cascade's shape). A host needing per-op,
per-input-leg rules (the portal's product/relation escalation) still writes its
own KeyRule; this generic one covers the :identity | :all case.