Deduplicated label for build steps

Build logs now show a "Deduplicated" label on steps that waited for another parallel build to compute the same result.

When you run multiple builds simultaneously that share identical steps, BuildKit automatically deduplicates the work — one build computes the step while the others wait. Previously, these waiting steps appeared to be running normally. Now they display a "Deduplicated" badge so you can see exactly which steps benefited from this optimization.

This helps you understand your build performance and identify when parallel builds are efficiently sharing work.