Validate the request-shape concerns that do not depend on a target (path
form, unimplemented flags, mutually-exclusive output flags). Shared by
single-target resolution and group fanout so both reject the same shapes
before any network or CDB work. Returns the normalized attribute selection.
Validate the request-shape concerns that do not depend on a target (path form, unimplemented flags, mutually-exclusive output flags). Shared by single-target resolution and group fanout so both reject the same shapes before any network or CDB work. Returns the normalized attribute selection.