`dart build cli` promised layout

[dcharkes]

Current proposal: https://github.com/dart-lang/sdk/issues/60730#issuecomment-2904942492

What do we promise as layout for dart build exe?

If we change the directory layout later, we would break users that rely on the results. So we need to specify what users can expect as an output.

cc @mkustermann @bkonyi

Prerequisite for:

• https://github.com/dart-lang/sdk/issues/60889