feat: add aggregation metrics to evaluation score card

[IgnazioDS]

Summary

Adds additional aggregation metrics to the evaluation score card, addressing #637.

Currently the evaluation results page only shows the average of numeric scores. This PR adds:

• Median — robust central tendency, less sensitive to outliers
• Standard Deviation — measures score consistency/spread
• Min / Max — range boundaries
• Count — number of data points

These metrics are displayed in a compact grid below the existing average display, preserving the current UI hierarchy.

Design Decisions

• All metrics are universally applicable — no need to distinguish between error metrics, quality scores, or classifications. This avoids the complexity discussed in Support more aggregations of numeric eval output besides average and histogram #637 around RMSE semantics.
• Population std deviation (not sample) — since we're computing over the full evaluation run, not a sample.
• Zero-cost for empty results — all fields default to 0 when no valid scores exist.
• Comparison mode preserved — the primary average + comparison arrow UI is untouched; secondary metrics appear below.

Files Changed

File	Change
lib/evaluation/types.ts	Added minValue, maxValue, stdDeviation, medianValue, count to EvaluationScoreStatistics
lib/actions/evaluation/utils.ts	Extended calculateScoreStatistics() to compute all metrics
components/evaluation/score-card.tsx	Added secondary metrics grid below average display

Test Plan

• Evaluation with numeric scores shows all 6 metrics (avg, median, std dev, min, max, count)
• Evaluation with no scores shows avg = 0 and no secondary metrics grid
• Comparison mode (two evaluations) still renders correctly with arrow + percentage change
• Metrics are accurate: verify against manual calculation on a small dataset

Addresses #637

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Note

Low Risk
Low risk UI/data-display change that extends computed score statistics; main risk is any downstream code expecting EvaluationScoreStatistics to only contain averageValue.

Overview
Extends EvaluationScoreStatistics to include medianValue, stdDeviation, minValue, maxValue, and count, and updates calculateScoreStatistics() to compute these aggregates (including population std dev and median).

Updates the evaluation score card to render a compact secondary-metrics grid (median/std dev/min/max + count) beneath the existing average/comparison display, hiding the grid when there are no valid scores.

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[IgnazioDS]

@Rainhunter13 @skull8888888 This adds median, std dev, min, max, and count to the evaluation score card — addresses #637. All metrics are universally applicable (no semantic metadata needed). Happy to adjust the UI layout or add/remove metrics based on your feedback!