pub const FOREIGN_KIND_PREFIX: &str = "x-";Expand description
Prefix of a caller-declared check kind the engine records verbatim
and never interprets: x-<name>, name lowercase letters, digits
and hyphens. The prefix makes the declaration deliberate — a typo of
an engine kind cannot silently become a new kind — mirroring the
rule that a third ENGINE kind is a separate decision. Foreign kinds
never influence check_state; health lists them by count.