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.

docs/code/cli.md

@@ -193,19 +193,21 @@ is labelled with is the number your code actually drew from, instead of merely
 looking like it. A flow that declares no such input still records it, and
 nothing reads it. Sweep over seeds with `--param seed=1,2,3`.
 
-An input declared as an `artifact` takes the file a previous run produced,
-named rather than typed out:
+Any input takes what a previous run produced, named rather than typed out —
+a checkpoint, but equally a config object nobody wants to paste into a shell:
 
 ```sh
 fluksio run evaluate --dataset @run:1758042000123-9f2ab41c.dataset
 fluksio run evaluate --dataset sha256:6dd1f0…
+fluksio run train --meta @run:1758042000123-9f2ab41c.dataset_meta
 ```
 
-`@run:<id>.<output>` is what that run's output was, and a bare digest is the
-content itself; the engine resolves either into the reference. Passing the
-whole reference as JSON still works and is what a script that already holds one
-does — which is the same thing `flow.submit(dataset=run.result["dataset"])`
-does from Python.
+`@run:<id>.<output>` is whatever that run's output was, whole and with its own
+type; a bare digest is the content itself, resolved into a reference. Both
+spellings are reserved on every input, `str` included, so an input that has to
+carry one of them literally cannot. Passing the value as JSON still works and
+is what a script that already holds one does — the same thing
+`flow.submit(dataset=run.result["dataset"])` does from Python.
 
 Run a flow with no parameters at a terminal and it asks for them, one line per
 declared input, with the declared value in brackets: