Partial Build

It should be relatively simple to get the list of touched files/folders from a PR. Then, if the touched folders all match the pattern of something like ^(legacy|release|experimental)/([a-z]+)/(\2)[a-z0-9_]+ -- that is, the expected folder layout -- run the build only on the touched folders.

If any other files are touched, run a full build, because the PR may include tooling updates etc.

Note 1: deletions or modifications to release/shared or experimental/shared would also require a full build. New files in those folders can only be referenced by changes to a package, so we don't need to do a full build if only new files are found there.

Note 2: release/packages would require a full build of release/

GitHub Action

Second part to this is to do this in a GHA so that the build can run automatically on the contributor's account, avoiding the need to involve TC build altogether.