Stage caching for batch runs, and an engine that lives in the command

A code node in a batch run is now fingerprinted by its source, its raw
settings and the values it reads — an artifact input counting as its digest,
which is what the content addressing was always for. A run that finds the key
restores what the earlier one returned and skips the node, recorded as
`cached`. The run history is the cache: `run_node.outputs` beside the
`cache_key` the schema already had, no second store. On for code nodes, never
for the built-in and connector types that have side effects; off per node with
`@node(cache=False)` and per run with `--no-cache`.

Emissions are not replayed on a hit, so a cached training node returns its
result without redrawing its curve. Recorded in NOTEPAD.md with the two other
deliberate limits.

`fluksio run --local` boots the real app in the command's own process and
drives it through its ASGI interface behind the ordinary client, so a run no
longer needs a `serve` terminal beside it — same data directory, same history,
and the cache carries between the two. It always waits, because the engine it
starts lives exactly as long as the command.

Also: `fluksio sweep --param lr=0.1,0.01` for the product of the lists,
`run --follow` for a run's numbers as they arrive, Ctrl-C cancelling a waited
run rather than abandoning it, coloured statuses on a terminal, and `name`
made optional on the metrics endpoint so a follower can ask for every series.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/nodes/base.py

@@ -91,6 +91,7 @@ class Node:
         "synchronous",
         "_on_health",
         "supervisor",
+        "fingerprint",
     )
 
     def __init__(
@@ -107,6 +108,10 @@ class Node:
         # Set by the controller before start(); a node built on its own (tests,
         # previews) runs its loops unsupervised.
         self.supervisor: Supervisor | None = None
+        # Everything about this node a cached result would depend on, hashed.
+        # Empty means it is not cacheable — which is every node type that is
+        # not code, and every node built for anything but a run.
+        self.fingerprint: str = ""
         self.params = dict(params) if params else {}
         self.synchronous = bool(self.params.get("synchronous", False))