feat(lsp): add completions for type system keywords

Added top-level keyword completions for:
- concept: with semicolon-terminated snippet
- sub_concept: with dot notation snippet
- concept_comparison: with brace-delimited snippet

Added test verifying type system keywords appear in
completions with correct snippet formatting.

src/lsp/completion_tests.rs

@@ -248,6 +248,57 @@ character Bob {}"#;
         }
     }
 
+    #[test]
+    fn test_type_system_completions() {
+        let doc = Document::new("".to_string());
+        let params = make_params(0, 0);
+
+        let result = completion::get_completions(&doc, &params);
+        assert!(result.is_some());
+
+        if let Some(response) = result {
+            match response {
+                | tower_lsp::lsp_types::CompletionResponse::Array(items) => {
+                    // Should have type system keywords
+                    assert!(
+                        items.iter().any(|item| item.label == "concept"),
+                        "Should have concept keyword"
+                    );
+                    assert!(
+                        items.iter().any(|item| item.label == "sub_concept"),
+                        "Should have sub_concept keyword"
+                    );
+                    assert!(
+                        items.iter().any(|item| item.label == "concept_comparison"),
+                        "Should have concept_comparison keyword"
+                    );
+
+                    // Check snippets have correct format
+                    let concept_item = items.iter().find(|item| item.label == "concept");
+                    assert!(concept_item.is_some());
+                    if let Some(item) = concept_item {
+                        let snippet = item.insert_text.as_ref().unwrap();
+                        assert!(
+                            snippet.contains(";"),
+                            "concept snippet should end with semicolon"
+                        );
+                    }
+
+                    let sub_item = items.iter().find(|item| item.label == "sub_concept");
+                    assert!(sub_item.is_some());
+                    if let Some(item) = sub_item {
+                        let snippet = item.insert_text.as_ref().unwrap();
+                        assert!(
+                            snippet.contains("."),
+                            "sub_concept snippet should contain dot notation"
+                        );
+                    }
+                },
+                | _ => panic!("Expected array response"),
+            }
+        }
+    }
+
     #[test]
     fn test_no_duplicate_completions() {
         let source = "character Alice {}\ncharacter Alice {}"; // Duplicate name