Artifacts: bytes a node produced, addressed by their content

A checkpoint is not a message. DType.ARTIFACT carries a reference — digest,
size, media type, name — so everything on the wire stays JSON and thirty
megabytes never sit in Redis, which answers the vision's open binary-payload
question by narrowing it: inline codecs would only serve payloads too small to
be worth a round trip, and nothing asks for that.

The store is content-addressed rather than per-run, for three reasons that all
pay later: a sweep whose fifty configs share one preprocessed input stores it
once, a reference stays valid however it is passed around because it names
content instead of a location, and the digest is what a stage cache will
compare — so building it in now is what keeps that from being a change to the
message contract.

Node code calls fluksio.save_artifact/load_artifact and cannot tell whether it
is writing the engine's own directory or putting bytes over HTTP, which is
what will let the same flow run on a remote worker unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AD8SfVhzXBG2nAfFcVh3iD

frontend/src/components/Flow/NodePanel.tsx

@@ -59,6 +59,7 @@ const DTYPES: DType[] = [
   "series",
   "record",
   "list",
+  "artifact",
 ]
 
 /** What a list may hold. One declared level: no list of lists. */
@@ -805,6 +806,9 @@ const PLACEHOLDER: Record<DType, string> = {
   series: '{"lines": []}',
   record: "{}",
   list: "[]",
+  // Bytes never travel as a message: the node stores them and returns what
+  // the next one opens.
+  artifact: 'fluksio.save_artifact(b"", "result.bin")',
 }
 
 const SCAFFOLD_DOC =