Follow a record into its fields, name the metrics, name the version

Three things the first export pass got wrong for a real study.

**Dotted paths.** A node returns a record, not a scalar — the numbers arrive
inside `final_metrics` — so `--metrics final_metrics.train_loss` yielded an
empty column and `--metrics final_metrics` yielded the whole record in one
cell. Both sides of the wide table now take dotted paths, and the defaults
reach the same depth: every number a result carries is a column named by its
path, and inputs are compared leaf by leaf, so two configurations differing in
one field give that field as the axis rather than two blobs that are merely
not equal. Lists stay whole — a curve belongs in the long table.

**`--list`.** Metric names are flow-qualified, so `--name train_loss` matched
nothing and said only that. `fluksio export metrics --list` prints the names
the selection carries, and an empty export made with `--name` points at it.

**A version to compare.** The CLI ships ahead of the engine and a stale one
answered a flat 404 with nothing anywhere in the API to tell how old it was.
The engine reports `version` on `/observability/summary`, `fluksio status`
prints it, and a 404 from export now names both versions — or says "older"
when the field itself predates the engine. Bumped to 0.1.5, which is what
makes the number worth reading.

Also formats `flow/metrics.py`, which had been committed unformatted and was
the last `ruff format --check` failure.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A9Hdrmf2cwNABCnE5x9UJa

docs/code/api.md

@@ -129,7 +129,9 @@ Both exports stream `csv` (the default) or `jsonl`, and take the selection the
 history takes plus `?ids=a,b,c`, `?since=` and `?until=`. `export/runs` puts
 the inputs that *vary* across the selection in `param.` columns — the sweep
 axis — unless `?params=` names them, and the run's numbers in `metric.`
-columns. The run id is on every row of both, which is what makes an exported
+columns. Both are dotted paths into whatever a node returned, so
+`?metrics=final_metrics.train_loss` selects a field of a record and the
+defaults reach every number inside one. The run id is on every row of both, which is what makes an exported
 file a join back to the run rather than a loose table.
 
 `compare` answers in the same shape a chart widget draws, so three training
@@ -186,7 +188,7 @@ read one back.
 
 | Path | What |
 |---|---|
-| `/observability/summary` | engine health — always 200, degraded or not |
+| `/observability/summary` | engine health — always 200, degraded or not — and the `version` it is running |
 | `/observability/timeseries` | executions and failures over a window |
 | `/observability/flows` | per-flow rollups with a 60-slice trend |
 | `/observability/runs` | recent cascades, with `?flow=`, `?since=`, `?until=` |