feat: add config validation and dependency cycle detection

- Config::validate() checks project.root, language, scan.include,
  python.src_roots, caching.max_cache_age, and scan.max_file_size
- Add parse_duration() and parse_file_size() helper functions
- Implement DFS-based cycle detection in cycle_detector.rs
- Wire cycle detection into renderer critical points section
- Add comprehensive unit tests for all new functionality

archdoc-core/src/renderer.rs

@@ -3,6 +3,7 @@
 //! This module handles generating Markdown documentation from the project model
 //! using templates.
 
+use crate::cycle_detector;
 use crate::model::ProjectModel;
 use handlebars::Handlebars;
 
@@ -493,7 +494,14 @@ impl Renderer {
         let data = serde_json::json!({
             "high_fan_in": high_fan_in,
             "high_fan_out": high_fan_out,
-            "cycles": Vec::<String>::new(), // TODO: Implement cycle detection
+            "cycles": cycle_detector::detect_cycles(model)
+                .iter()
+                .map(|cycle| {
+                    serde_json::json!({
+                        "cycle_path": format!("{} → {}", cycle.join(" → "), cycle.first().unwrap_or(&String::new()))
+                    })
+                })
+                .collect::<Vec<_>>(),
         });
         
         // Create a smaller template just for the critical points section