fix: remove std::path from application layer, document merge error behavior

crates/application/src/use_cases/build_code_graph.rs

@@ -53,6 +53,8 @@ where
                 let mut graph = result.graph().clone();
                 if level == DiagramLevel::Module {
                     if let Some(pa) = self.project_analyzer {
+                        // Propagate error: a present but malformed manifest is a user error,
+                        // not a silent skip.
                         let project_cg = pa.analyze(root)?;
                         graph.merge_project_edges(&project_cg);
                     }

crates/application/src/use_cases/generate_diagram.rs

@@ -44,8 +44,7 @@ fn render_split(
     let ext = overview
         .files()
         .first()
-        .and_then(|f| std::path::Path::new(f.name()).extension())
-        .and_then(|e| e.to_str())
+        .and_then(|f| f.name().rsplit_once('.').map(|(_, e)| e))
         .unwrap_or("txt");
 
     if let Some(f) = overview.files().first() {