Platform prefixes for multi-architecture bake builds

Build logs for multi-architecture depot bake builds now display platform prefixes, so you can identify which architecture ran each build step.

When your bake file targets multiple platforms (for example, linux/amd64 and linux/arm64), log lines now show prefixes like [frontend linux/amd64 1/4] instead of [frontend 1/4]. This helps you quickly identify architecture-specific issues when debugging multi-platform builds.

Single-architecture builds remain clean and prefix-free to avoid unnecessary noise.