refactor(scatter): move logging call into pipeline closure

Add an explicit `PathBuf` type annotation and consolidate the "indexing" info log with the chunk-opening logic. This reduces pipeline boilerplate by keeping the logging directly in the initial stage closure.

src/obikmer/src/steps/scatter.rs

@@ -22,11 +22,9 @@ pub fn scatter(
     use obikseq::RoutableSuperKmer;
 
     let t = Stage::start("scatter");
-    let path_source = path_source.inspect(|p| info!("indexing: {}", p.display()));
-
     let pipe = obipipeline::make_pipe! {
         PipelineData : PathBuf => Vec<RoutableSuperKmer>,
-        ||? { |path| open_chunks(path) }                                               : Path     => RawChunk,
+        ||? { |path: PathBuf| { info!("indexing: {}", path.display()); open_chunks(path) } } : Path => RawChunk,
         |?  { move |rope| obiread::normalize_sequence_chunk(rope, k) }                : RawChunk => NormChunk,
         |   { move |rope| obiskbuilder::build_superkmers(rope, k, level_max, theta) }: NormChunk => Batch,
     };