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/sdk/client.py

@@ -428,9 +428,9 @@ class Client:
     ) -> list[dict[str, Any]]:
         """One row per run: its inputs as columns, its final numbers, its code.
 
-        The arm-comparison table. The inputs kept are the ones that vary
-        across the selection unless ``params`` names them, which is the axis a
-        sweep is read along.
+        The arm-comparison table. Every input the selection recorded is a
+        column, so the schema does not depend on which runs were asked for;
+        ``params`` narrows it to the axis a sweep is read along.
         """
         query: dict[str, Any] = {}
         if params: