resolveStigSingleModule
Resolves a single-element modules: ListProperty<String> input to a StigTargetModule.
Throws NotImplementedError when modules is empty (no default fallback yet) or contains more than one entry (multi-module pipeline not implemented). Both error messages explain how to set the property correctly.
Future: support modules.size == 0 (operate on all stig subprojects) and modules.size > 1 (multi-module pipeline) once individual task implementations gain the necessary plumbing.