Build number calculation has been reworked, and now always uses version tags.
Version tags are now platform-agnostic, and have the form vX.Y.Z-BUILD.
Running rt archive or rt submit will now check first for the presence of a version tag at the HEAD commit, and will refuse to run if there isn't one.
Running rt tag will create a new version tag. The version will be assumed to be the same as the previous tag, unless it is explicitly provided with the --explicit-version option. The build number will be incremented from the previous tag. If old platform-specific tags are found, they are included in the calculation to find the newest build number.
The rt tag command will refuse if there's already a tag.