test: document what the zero-constructor detector does not catch

It requires a literal, so `idle: DEFAULT_IDLE` escapes even though a named
constant fabricates exactly as effectively as `100.0`. It flags `false` and
not `true`, because a struct of flags set true is usually a builder and
flagging every one would bury the signal.

Worth writing down because the failure mode of this file is someone reading
a pass as a proof that no fabricating constructor exists. It narrows 150
constructors to a list a person can read; the four it finds were confirmed
by reading them, and it certifies nothing about the rest.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>