EphemerisSamplingError and ManeuverApplicationError were missing the
attribute the other fifteen error kinds carry. Record the distinction
between an error enum, which is marked so a new category does not break a
caller, and a domain state enum, which is left exhaustive so the compiler
points at every match that a new variant needs.
apsis-core described Error and Result as the workspace's single error
surface, but no crate uses them: every layer with a domain specific failure
defines its own Error and ErrorKind, whose variants name the concrete
conditions a caller matches on. Correct the doc comment and record the
strategy, including the Option versus Result split and the tolerance
conventions.