refactor: deepen modules, consolidate inference, delete dead code

- Extract build_graph/load_config/create_renderer in presentation (393→~250 lines)
- Move module inference into ModuleName::from_path(), delete 3 scattered copies
- Move resolve_relationships/filter_external_imports into CodeGraph
- Add LanguageExtractor trait in tree-sitter adapter
- Add CodeGraph::elements_by_module(), replace 6 identical grouping loops
- Delete dead RenderDiagrams query

crates/application/tests/render_diagrams_tests.rs

@@ -1,78 +0,0 @@
-mod fakes;
-
-use archlens_application::queries::RenderDiagrams;
-use archlens_domain::{
-    CodeElement, CodeElementKind, CodeGraph, FilePath, ModuleName, OutputConfig,
-};
-
-use fakes::{FakeDiagramRenderer, FakeOutputWriter};
-
-fn build_graph() -> CodeGraph {
-    let mut graph = CodeGraph::new();
-    graph.add_element(
-        CodeElement::new(
-            "OrderService",
-            CodeElementKind::Class,
-            FilePath::new("src/service.rs").unwrap(),
-            1,
-        )
-        .unwrap()
-        .with_module(ModuleName::new("Orders").unwrap()),
-    );
-    graph.add_element(
-        CodeElement::new(
-            "BillingService",
-            CodeElementKind::Class,
-            FilePath::new("src/billing.rs").unwrap(),
-            1,
-        )
-        .unwrap()
-        .with_module(ModuleName::new("Billing").unwrap()),
-    );
-    graph
-}
-
-#[test]
-fn renders_single_diagram_and_writes_output() {
-    let renderer = FakeDiagramRenderer::new();
-    let writer = FakeOutputWriter::new();
-    let config = OutputConfig::default();
-
-    let use_case = RenderDiagrams::new(renderer, writer);
-    use_case.execute(&build_graph(), &config).unwrap();
-
-    let outputs = use_case.writer().written_outputs();
-    assert_eq!(outputs.len(), 1);
-    assert_eq!(outputs[0].files().len(), 1);
-}
-
-#[test]
-fn split_mode_renders_overview_plus_per_module_diagrams() {
-    let renderer = FakeDiagramRenderer::new();
-    let writer = FakeOutputWriter::new();
-    let config = OutputConfig::default().with_split_by_module(true);
-
-    let graph = build_graph(); // has 2 modules: Orders, Billing
-
-    let use_case = RenderDiagrams::new(renderer, writer);
-    use_case.execute(&graph, &config).unwrap();
-
-    let outputs = use_case.writer().written_outputs();
-    // 1 overview + 2 module diagrams = 3 writes
-    assert_eq!(outputs.len(), 3);
-}
-
-#[test]
-fn empty_graph_still_produces_output() {
-    let renderer = FakeDiagramRenderer::new();
-    let writer = FakeOutputWriter::new();
-    let config = OutputConfig::default();
-
-    let graph = CodeGraph::new();
-
-    let use_case = RenderDiagrams::new(renderer, writer);
-    use_case.execute(&graph, &config).unwrap();
-
-    let outputs = use_case.writer().written_outputs();
-    assert_eq!(outputs.len(), 1);
-}