Event Deltas: Heatmap hover tooltip with percentile context #1911

Summary

The heatmap tooltip currently shows the raw value and count for the hovered cell. Add percentile context — e.g., "423ms (p97.3)" — so users immediately understand where a latency value falls in the overall distribution without mental math.

This could be computed client-side from the bucket data already available (cumulative count up to the hovered bucket / total count).

Dependencies

• HDX-3697 (heatmap overhaul)