Export every recorded input, not only the ones that vary

`export runs` dropped a `param.*` column whose value was constant across the
exported runs, so a downstream filter broke depending on which runs the
selection happened to hold. Every input the selection recorded is a column
now; `--params` still narrows it to a sweep's axis. The metrics default is
unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/fluksio/api/routes/runs.py

@@ -422,25 +422,6 @@ def _dig(record: dict[str, Any], path: str) -> Any:
     return value
 
 
-def _varying(runs: list[Run]) -> list[str]:
-    """The inputs that differ across these runs — the axis of a sweep.
-
-    What a reader comparing arms wants as columns, compared leaf by leaf: two
-    configurations differing in one field give that field as a column rather
-    than two blobs that are not the same. Under two runs nothing can differ,
-    and a table of one run with none of its inputs in it is not worth reading,
-    so all of them are kept.
-    """
-    keys = sorted({path for run in runs for path, _ in _leaves(run.params)})
-    if len(runs) < 2:
-        return keys
-    return [
-        key
-        for key in keys
-        if len({json.dumps(_dig(run.params, key), sort_keys=True) for run in runs}) > 1
-    ]
-
-
 def _scored(runs: list[Run]) -> list[str]:
     """A run's final numbers: every number its declared outputs carry, however
     deep it sits. A flag is not a number, and neither is a label."""
@@ -557,9 +538,9 @@ def export_runs(
     """One row per run: what it was given, what it scored, what code it ran.
 
     The arm-comparison table. Inputs are columns rather than one JSON blob —
-    by default the ones that vary across the selection, which is the sweep
-    axis; ``params`` names them instead. ``metrics`` narrows the final numbers
-    to a few of a run's declared outputs.
+    every input the selection recorded, so the schema is the same whichever
+    runs are asked for; ``params`` narrows it. ``metrics`` narrows the final
+    numbers to a few of a run's declared outputs.
 
     Both take dotted paths into a record a node returned:
     ``metrics=final_metrics.train_loss,test_metrics.known.perfect`` selects
@@ -567,7 +548,9 @@ def export_runs(
     depth.
     """
     runs = _selected(session, flow, status, group, ids, since, until)
-    inputs = [part for part in params.split(",") if part] or _varying(runs)
+    inputs = [part for part in params.split(",") if part] or sorted(
+        {path for run in runs for path, _ in _leaves(run.params)}
+    )
     scores = [part for part in metrics.split(",") if part] or _scored(runs)
     columns = [
         *RUN_COLUMNS,