docs(dev/guide): tighten unit-tests section per review

Author: claude

CONTRIBUTING.md

@@ -348,11 +348,11 @@ ExitCode::from(match self {
 
 ## Unit Tests
 
-A unit-test module may either sit inline as `mod tests { ... }` in its parent or live in a dedicated external `tests` submodule. The inline form is appropriate for short test modules; once the block grows long enough to noticeably extend the length of the parent and get in the way of reading the rest of the module, move the tests into an external file.
+A unit-test module may either sit inline as `mod tests { ... }` in its parent or live in a dedicated external `tests` submodule. Use the inline form for short test modules. Once the block becomes long enough to obscure the surrounding module, move the tests into an external file.
 
 ### When the inline form is acceptable
 
-There is nothing wrong with `mod tests { ... }` by itself. Reserve the inline form for modules whose entire test suite fits in a small number of lines, so that the block does not noticeably extend the length of the parent. The problem that this convention addresses is a long inline block that extends the length of the main module and makes it harder to traverse. Use the number of lines as the deciding factor: once the inline tests grow long enough to get in the way of reading the rest of the module, move them into an external file.
+The inline form `mod tests { ... }` is acceptable on its own. Reserve it for modules whose entire test suite fits in a small number of lines, so the block does not noticeably extend the parent. Use the number of lines as the deciding factor.
 
 ### Where the external file sits