Where StructuredTaskScope Ends: Building the Flow Layer in Exeris

This is the fifth article in the Exeris Kernel series. The series has been building one coherent architectural picture: article one replaced ThreadLocal with ScopedValue on the context propagation path; article two argued that some layers do not benefit from runtime polymorphism; article three showed where StructuredTaskScope genuinely earns its keep; article four pushed TLS off the heap entirely. This one is about where STS stops being the right tool — and what had to be built instead.