Add ArchUnit guards for Java multi-frontend boundaries in a test-only
module. Identify PBS by frontend-module origin, allow only the registry
as composition root, and lock the wording in specs 19, 20, and 22.
Housekeep DSC-0064 with LSN-0066.
Publish the language-versus-platform validation matrix in compiler-general
specs, retitle spec 19, and recut PBS diagnostics so common structural
failures are not PBS coverage.
Housekeep DSC-0060 with LSN-0063.
