A cached node keeps its curve, and any input can name a run's output

A cache hit still replays no emissions — those values were the story of an
execution that is not happening — but the run they were recorded in is now
written on the row (`run_node.cached_from`), and the metrics endpoints read the
series back from there. So a reused run answers `run.metrics("train.loss")`
with the same points the run that trained did, rather than looking like a run
that produced no numbers at all. Pointed at rather than copied: a sweep of 500
reusing one frozen node would otherwise duplicate its curve 500 times.

That needed the cross-flow restore fixed first. The cache key has no flow in
it while the stored outputs are named for the flow that produced them, so
`quick.prepare` getting a hit from `train` wrote `train.dataset` into `quick`'s
state and the next node was called without its argument. One rule now covers
both halves: `requalify` reads a name owned by one flow as the same name in
another, applied to the restored outputs, to the node id behind the pointer,
and to the series names on the way out. Reuse across flows is kept.

Also: `@run:<id>.<output>` and a bare `sha256:` digest resolve on every input,
not only artifacts. Chaining a run's json config into the next one from a shell
meant pasting the whole object inline, and the CLI could not even send the
spelling — `_coerce` died in `json.loads` before the engine saw it. Both
spellings are reserved on every input now, `str` included, and `_from_run`
returns whatever the run's result holds rather than only a reference.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dp9L6gakMVro1K2C5zdtBE

docs/reference/payload-types.md

@@ -99,12 +99,6 @@ stays valid wherever the store is reachable from, including on another machine.
 Node code produces one with `fluksio.save_artifact` and opens one with
 `fluksio.load_artifact`. See [Writing node code](../code/nodes.md#bytes-artifacts).
 
-As a *run parameter* it is also accepted as text, since nobody wants to paste
-the object into a shell: `@run:<id>.<output>` names what a run produced, and a
-bare `sha256:…` digest names the content. Both resolve to the reference above
-before the run starts, so the CLI, the run dialog and a python caller all mean
-the same thing.
-
 ### `json`
 
 Anything JSON-serializable. The escape hatch, and the right answer when a
@@ -113,6 +107,18 @@ payload genuinely has no fixed shape.
 Reach for it last. A `json` port tells the canvas, the widget picker and the
 next author nothing.
 
+## Naming a run's output
+
+Any *run parameter* is also accepted as text, since nobody wants to paste an
+object into a shell. `@run:<id>.<output>` names what an earlier run produced —
+whatever its type, an `artifact` reference or a `json` config alike — and a bare
+`sha256:…` digest names content in the artifact store. Both resolve before the
+run starts, so the CLI, the run dialog and a python caller all mean the same
+thing by the same string.
+
+Both spellings are reserved on every input, `str` included: an input that has to
+carry one of them literally is asking for a value this engine reads as a name.
+
 ## What a widget will bind to
 
 | Widget | Accepts |