refactor: five architectural deepening improvements

Candidate 1 (NormalizedGraph): qualify→resolve→filter is now a single
named operation returning a distinct type; raw CodeGraph cannot call
module_edges/subgraph_by_module — pipeline order enforced at compile time.

Candidate 2 (Use cases): GenerateDiagram, CheckFreshness, DiffDiagram
extracted to application/src/use_cases/; presentation is now a thin CLI
dispatcher (~100 lines less, three fewer local functions).

Candidate 3 (ExtractionContext): shared accumulator for both Rust and
Python extractors replaces parallel Vec<> + 4-arg passing chains.

Candidate 4 (ModuleAssignment): ModuleName::assign() returns
ModuleAssignment { Explicit | Inferred | Unresolved } instead of Option,
callers can distinguish resolution strategies.

Candidate 5 (SplitRenderer): append_cross_module_deps removed from
DiagramRenderer port; replaced by render_for_module() default impl —
port interface now reflects what all renderers actually share.

crates/adapters/mermaid/src/mermaid_renderer.rs

@@ -226,42 +226,36 @@ impl DiagramRenderer for MermaidRenderer {
         Ok(RenderOutput::single(file))
     }
 
-    fn append_cross_module_deps(
+    fn render_for_module(
         &self,
-        content: &str,
+        subgraph: &CodeGraph,
         module: &ModuleName,
-        deps: &[(ModuleName, usize)],
-    ) -> String {
-        if deps.is_empty() {
-            return content.to_string();
-        }
-
-        let src_id = format!(
-            "{}_module",
-            module.as_str().to_lowercase().replace('-', "_")
-        );
-        let mut extra = format!(
-            "    class {src_id}[\"{}\"] {{\n        <<module>>\n    }}\n",
-            module.as_str()
-        );
-
-        for (dep_mod, count) in deps {
-            let dep_id = format!(
-                "{}_module",
-                dep_mod.as_str().to_lowercase().replace('-', "_")
+        cross_deps: &[(ModuleName, usize)],
+    ) -> Result<RenderOutput, DomainError> {
+        let base = self.render_class_diagram(subgraph);
+        let content = if cross_deps.is_empty() {
+            base
+        } else {
+            let src_id = format!("{}_module", module.as_str().to_lowercase().replace('-', "_"));
+            let mut extra = format!(
+                "    class {src_id}[\"{}\"] {{\n        <<module>>\n    }}\n",
+                module.as_str()
             );
-            extra.push_str(&format!(
-                "    class {dep_id}[\"{}\"] {{\n        <<module>>\n    }}\n",
-                dep_mod.as_str()
-            ));
-            let label = if *count == 1 {
-                "1 dep".to_string()
-            } else {
-                format!("{count} deps")
-            };
-            extra.push_str(&format!("    {src_id} --> {dep_id} : {label}\n"));
-        }
-
-        format!("{content}\n{extra}")
+            for (dep_mod, count) in cross_deps {
+                let dep_id = format!(
+                    "{}_module",
+                    dep_mod.as_str().to_lowercase().replace('-', "_")
+                );
+                extra.push_str(&format!(
+                    "    class {dep_id}[\"{}\"] {{\n        <<module>>\n    }}\n",
+                    dep_mod.as_str()
+                ));
+                let label = if *count == 1 { "1 dep".to_string() } else { format!("{count} deps") };
+                extra.push_str(&format!("    {src_id} --> {dep_id} : {label}\n"));
+            }
+            format!("{base}\n{extra}")
+        };
+        let file = RenderedFile::new("diagram.mmd", &content)?;
+        Ok(RenderOutput::single(file))
     }
 }