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/code/cli.md

@@ -304,6 +304,36 @@ flow's inputs, the same as `run`'s are, and `--seed`, `--no-sync`,
 `--no-cache` and `--local` mean what they do there. `--wait` blocks until all
 of them are finished and exits non-zero if any failed.
 
+### `fluksio export`
+
+```sh
+fluksio export metrics --flow train --name train_loss,val_loss --stride 10 -o curves.csv
+fluksio export runs    --flow train --status ok > arms.csv
+```
+
+The two tables an analysis reads. `export metrics` is the long one — a row per
+run, metric and step — which is what a plotting library takes without
+reshaping; `--name` keeps the metrics it lists and `--stride` keeps every Nth
+point of *each* curve. `export runs` is the wide one: a row per run with its
+inputs as columns, its final numbers, its status, its duration and the commit
+and digest of the code it ran.
+
+The inputs that become columns are the ones that **vary** across the selected
+runs — the axis of the sweep, which is what a comparison is read along —
+unless `--params lr,seed` names them. `--metrics` narrows the final numbers
+the same way.
+
+Both take `--flow`, `--run ID` (repeat it), `--group`, `--status`, `--since`,
+`--until` and `--local`, and both put the run id on every row: it is the join
+back to the run page and to what the run made.
+
+`--format` is `csv` (the default), `jsonl` or `parquet`; output goes to stdout
+unless `-o FILE` names somewhere. Parquet keeps the types and needs pyarrow —
+`pip install 'fluksio[parquet]'` — and a file to write, since it is not a
+stream. In a notebook, `Client.export_metrics()` and `Client.export_runs()`
+answer the same rows as a list of dicts, which `pandas.DataFrame` takes
+directly.
+
 ## What lives in the data directory
 
 ```text