Small status update: I received some excellent feedback off-thread and will be preparing a Pitch #2 soon.
It will extend the semantics to also cover inheritance of values with detached tasks, although with very carefully specialized semantics to avoid infinitely growing task local bound value stacks. It'll help with end-to-end tracing even in presence of detached tasks.
It also means we pretty definitely do need the Key type, as we'll need to special mark some keys for storage in "always inherit" limited-space storage.
Thanks for the input and cya in Pitch #2 soon