Found by serving the include evidence: `tools.md` beside a bundle's index was
itself scanned as a bundle, so a file meant only to be included took a URL of its
own, appeared in its section's listing, and turned the including bundle into a
one-member series. The real binary showed the phantom series nav; no test would
have, because every fixture happened to name its partials differently.
The rule mirrors the one directories already have, `_index` excepted since that
names its directory. `{{< include file="_tools.md" >}}` is now the shape to write.
bdeshi
5e574b038f