Export runs and their curves as tables an analysis reads

`fluksio export metrics` is the long table — a row per run, metric and step —
and `fluksio export runs` the wide one, a row per run with the inputs that
*vary* across the selection as columns beside its final numbers, status,
duration and the commit and digest of the code it ran. Both carry the run id
on every row, which is the join back to the run page and what makes an
exported file auditable. `Client.export_metrics`/`export_runs` answer the same
rows to a notebook.

The engine streams csv or jsonl from two routes declared above `/{run_id}`;
parquet is a client-side conversion behind the new `fluksio[parquet]` extra,
so nobody pays for pyarrow who does not want dtypes kept. The long export
reads each run through `_series`, so a cached node's curve comes with it, and
`--stride` thins each series rather than the concatenation of all of them.

Two things they needed on the way: `GET /runs` takes `?since=` and `?before=`,
so a long history pages by the last row's own timestamp instead of an offset
that shifts under it; and a read that reaches no engine now says so in half a
second rather than seven, because `runs`, `flavors`, `export` and an unwatched
`status` pass `retries=0`. Everything that submits keeps them.

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

docs/concepts/runs.md

@@ -318,6 +318,28 @@ an epoch, or samples seen — joined on the step the two share.
 One run in full is params, the per-node record with its logs and traceback,
 the artifacts it made, its metrics and its result.
 
+### Taking it into a dataframe
+
+An analysis wants a table rather than a screen, and there are two it usually
+wants. `fluksio export metrics` is the long one — a row per run, metric and
+step — and `fluksio export runs` is the wide one, a row per run with the
+inputs that varied as columns beside its final numbers:
+
+```python
+import pandas as pd
+from fluksio.sdk.client import Client
+
+client = Client()
+curves = pd.DataFrame(client.export_metrics(flow="train"))
+arms = pd.DataFrame(client.export_runs(flow="train", status="ok"))
+```
+
+The run id is on every row of both, so a curve joins to the arm it came from
+and to the run page it was recorded on, and the wide table carries the commit
+and the code digest — an exported file says what produced its numbers.
+[The CLI](../code/cli.md#fluksio-export) writes the same rows as csv, jsonl or
+parquet, which is where an export belongs: in the script beside the analysis.
+
 ### A dashboard, read against runs
 
 A run records values under the same names a dashboard binds to — a run of