vf-tui: compare for all metrics

[snimu]

Description

vf-tui now is always in group mode when we enter compare model (previously, it went v->g, now it's just v). There:

• hitting enter on an arg will group by that arg's values, as before
• but we can now also select the avg-reward column
• hitting enter on it will allow us to select a different metric than the reward, and the table will update to show that metric's statistics

Pressing v will immediately get us into group mode:

Pressing enter on any of the arg columns still allows us to group by the unique values of that arg:

But now, it's also possible to select the "avg reward" column (renamed from just "avg"):

Pressing enter here doesn't group by the unique values (because that doesn't make any sense). Instead, it allows us to select a different metric:

And then selecting one will show the results of that metric like the reward before:

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

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Note

Medium Risk
Adds new metric-selection and bucketing logic to the comparison TUI, changing how comparison tables are computed and navigated; main risk is UI/regression in table layout and metric aggregation/formatting across diverse value ranges.

Overview
Extends the run comparison TUI to compare any numeric metric, not just reward, via a new Select dropdown and a metric-aware avg/min/max + distribution “mix” display.

Refactors comparison navigation to a single cursor model (left/right/enter) that toggles grouping on setting columns or opens the metric selector when the metric column is selected, and updates copy/export to reflect the chosen metric.

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