Closes the payload loop for a resource-backed node: writes a combinator's output row into the node's OWN resource (cell.meta.resource), keyed by the cell key.

This is what makes "the resource IS the node, and its rows ARE its payload" true in the code, not just the docs. A reduce/join whose into returns a row and that omits an explicit upsert: has its row written HERE — an Ash upsert into the node's resource, with change-detection — so the common case needs no host write callback, and writing into a different resource becomes the explicit deviation (a custom upsert:), not the default.

The key attribute

The cell key (a string) maps to one resource attribute — the payload key. It defaults to :key; a resource whose primary key is named otherwise declares payload_key :flow_key in its reactive block. The row is written with that attribute set to the cell key; a :key field on the row itself is dropped (it's the coordination key, not a payload column).

Change detection

upsert/5 reads the existing row first and compares the writable attributes; it returns :changed only when a create or a real value change happened, so the drain's Op.put/parent-dirty only fires for genuine changes (a no-op recompute stays a no-op). Requires an upsert action on the resource — by default the action named :upsert with upsert?: true; override with payload_action.