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

backend/fluksio/models.py

@@ -397,6 +397,10 @@ class RunNode(SQLModel, table=True):
     #: Everything this node's output depends on, hashed: its source, its
     #: settings and the values it read. What a later run looks itself up by.
     cache_key: str = Field(default="", index=True, max_length=64)
+    #: The run this node's values were restored from, when it was not executed.
+    #: Its series lives there too, which is what makes a cached run's curve
+    #: readable without copying a few thousand rows per reuse.
+    cached_from: str = Field(default="", max_length=64)
     #: What it returned, as canonical JSON, so a node with this key can be
     #: skipped and its outputs restored. None when it may not be reused —
     #: opted out, too large, or a value JSON cannot carry. Written with