feat: implement all P1/P2/P3/P4 improvements from issue backlog

P1 correctness:
- filter test files by default (--include-tests to opt in)
- per-module diagrams show cross-module dependency arrows
- qualified type names (Module::TypeName) fix false edges from duplicate names

P2 output richness:
- method parameter types and return types in class diagrams (Rust + Python)
- Python pyproject.toml project analyzer (--level project for monorepos)

P3 unique value:
- boundary rules in archlens.toml ([rules] allow/deny, --strict enforcement)

P4 nice to have:
- dependency weight labels on module arrows (--no-weights to disable)
- --watch mode with 500ms debounce
- D2 renderer adapter (--format d2)
- interactive self-contained HTML viewer (--format html)
- git-aware incremental analysis (--since <ref>)

crates/domain/src/ports/diagram_renderer.rs

@@ -1,5 +1,15 @@
-use crate::{CodeGraph, DomainError, RenderOutput};
+use crate::{CodeGraph, DomainError, ModuleName, RenderOutput};
 
 pub trait DiagramRenderer {
     fn render(&self, graph: &CodeGraph) -> Result<RenderOutput, DomainError>;
+
+    fn append_cross_module_deps(
+        &self,
+        content: &str,
+        module: &ModuleName,
+        deps: &[(ModuleName, usize)],
+    ) -> String {
+        let _ = (module, deps);
+        content.to_string()
+    }
 }