Engine tests: clippy passes on Windows (Stills is Unix-only)

`Stills` is only used by the `#[cfg(unix)]` part of
`footage_on_other_computers_is_never_looked_at`. On Windows it was dead
code, so `cargo xtask ci` stopped at clippy (`-D warnings`).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/doc_path/tests.rs

@@ -34,9 +34,11 @@ impl Services for Recording {
     }
 }
 
-/// Probes every file as a still.
+/// Probes every file as a still. Only the Unix part of the test below imports files.
+#[cfg(unix)]
 struct Stills;
 
+#[cfg(unix)]
 impl crate::Importer for Stills {
     fn probe(&self, path: &str) -> Result<Footage, String> {
         if !std::path::Path::new(path).is_file() {