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

backend/tests/sdk/test_client.py

@@ -130,3 +130,25 @@ def test_a_run_that_is_gone_stops_the_wait_at_once(monkeypatch):
         handle.wait(poll=0)
     assert caught.value.status == 404
     assert len(calls) == 1, "a 404 is an answer, not a blip"
+
+
+def test_an_export_is_read_as_rows(monkeypatch):
+    """jsonl on the wire: a stream of documents, not one."""
+    seen = {}
+
+    def handler(request):
+        seen.update(request.url.params)
+        return httpx.Response(
+            200,
+            text='{"run": "r1", "step": 0}\n{"run": "r1", "step": 1}\n',
+            headers={"content-type": "application/x-ndjson"},
+        )
+
+    client = a_client(handler, monkeypatch)
+    rows = client.export_metrics(flow="train", ids=["r1", "r2"], status="ok")
+
+    assert seen["format"] == "jsonl"
+    assert seen["ids"] == "r1,r2"
+    assert seen["flow"] == "train"
+    assert seen["status"] == "ok"
+    assert [row["step"] for row in rows] == [0, 1]