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/alembic/versions/c9a5d1e73b48_stage_cache.py

@@ -0,0 +1,42 @@
+"""run_node.outputs, run.no_cache
+
+What a node returned, so a later run with the same cache key can skip it and
+restore its outputs instead of executing it again. `run.no_cache` is the
+per-run way to ask for the whole graph to run regardless.
+
+Revision ID: c9a5d1e73b48
+Revises: b7c21f4d9e30
+Create Date: 2026-08-24
+
+"""
+
+import sqlalchemy as sa
+import sqlmodel.sql.sqltypes
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "c9a5d1e73b48"
+down_revision = "b7c21f4d9e30"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "run_node",
+        sa.Column("outputs", sqlmodel.sql.sqltypes.AutoString(), nullable=True),
+    )
+    op.add_column(
+        "run",
+        sa.Column(
+            "no_cache",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.text("0"),
+        ),
+    )
+
+
+def downgrade():
+    op.drop_column("run", "no_cache")
+    op.drop_column("run_node", "outputs")