When trying to run dune build while another dune process already holds the build
directory lock, dune silently waits indefinitely with no indication to the user
what's happening. This creates a confusing experience where users don't know why
their build isn't progressing.

Desired behavior

Dune should inform the user that it's waiting for a lock held
by another process, and provide guidance on what to do if that process is stale.

Impact

• Users waste time waiting for builds that appear frozen
• Users may kill legitimate processes or create duplicate builds
• No clear guidance on how to resolve stale lock situations