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/application/src/use_cases/check_freshness.rs

@@ -0,0 +1,18 @@
+use archlens_domain::{DomainError, NormalizedGraph, ports::DiagramRenderer};
+
+/// Compares the current rendered output against an on-disk file.
+/// Returns `Ok(true)` if up to date, `Ok(false)` if stale.
+pub struct CheckFreshness<'a> {
+    pub graph: &'a NormalizedGraph,
+    pub renderer: &'a dyn DiagramRenderer,
+    pub existing_path: &'a std::path::Path,
+}
+
+impl<'a> CheckFreshness<'a> {
+    pub fn execute(&self) -> Result<bool, DomainError> {
+        let rendered = self.renderer.render(self.graph.as_graph())?;
+        let current = rendered.files().first().map(|f| f.content()).unwrap_or("");
+        let existing = std::fs::read_to_string(self.existing_path).unwrap_or_default();
+        Ok(current == existing)
+    }
+}

crates/application/src/use_cases/diff_diagram.rs

@@ -0,0 +1,46 @@
+use archlens_domain::{DomainError, NormalizedGraph, ports::DiagramRenderer};
+
+/// The semantic result of comparing two diagram snapshots.
+pub struct DiffResult {
+    pub added: Vec<String>,
+    pub removed: Vec<String>,
+}
+
+impl DiffResult {
+    pub fn is_empty(&self) -> bool {
+        self.added.is_empty() && self.removed.is_empty()
+    }
+}
+
+/// Compares the rendered current graph against an existing diagram file and
+/// returns which lines were added or removed.
+pub struct DiffDiagram<'a> {
+    pub graph: &'a NormalizedGraph,
+    pub renderer: &'a dyn DiagramRenderer,
+    pub existing_path: &'a std::path::Path,
+}
+
+impl<'a> DiffDiagram<'a> {
+    pub fn execute(&self) -> Result<DiffResult, DomainError> {
+        let rendered = self.renderer.render(self.graph.as_graph())?;
+        let current = rendered.files().first().map(|f| f.content()).unwrap_or("");
+        let existing = std::fs::read_to_string(self.existing_path).unwrap_or_default();
+
+        let current_lines: std::collections::HashSet<&str> = current.lines().collect();
+        let existing_lines: std::collections::HashSet<&str> = existing.lines().collect();
+
+        let added: Vec<String> = current_lines
+            .difference(&existing_lines)
+            .filter(|l| !l.trim().is_empty())
+            .map(|l| format!("+ {l}"))
+            .collect();
+
+        let removed: Vec<String> = existing_lines
+            .difference(&current_lines)
+            .filter(|l| !l.trim().is_empty())
+            .map(|l| format!("- {l}"))
+            .collect();
+
+        Ok(DiffResult { added, removed })
+    }
+}

crates/application/src/use_cases/generate_diagram.rs

@@ -0,0 +1,125 @@
+use std::path::PathBuf;
+
+use archlens_domain::{
+    BoundaryRule, DomainError, NormalizedGraph, RenderedFile, RenderOutput,
+    check_boundary_rules,
+    ports::DiagramRenderer,
+};
+
+/// Result of running the generate use case — exposed violations and any output
+/// that should be written to disk.
+pub struct GenerateDiagramResult {
+    pub violations: Vec<String>,
+    pub output: RenderOutput,
+}
+
+/// Orchestrates diagram generation: renders the graph (split or single),
+/// checks boundary rules, and returns the output for the caller to write.
+pub struct GenerateDiagram {
+    pub graph: NormalizedGraph,
+    pub renderer: Box<dyn DiagramRenderer>,
+    pub allow_rules: Vec<BoundaryRule>,
+    pub deny_rules: Vec<BoundaryRule>,
+    pub split_by_module: bool,
+    pub format_ext: String,
+    pub output_dir: Option<PathBuf>,
+}
+
+impl GenerateDiagram {
+    pub fn execute(self) -> Result<(), DomainError> {
+        // Boundary rule checking
+        let violations = if !self.allow_rules.is_empty() || !self.deny_rules.is_empty() {
+            check_boundary_rules(self.graph.as_graph(), &self.allow_rules, &self.deny_rules)
+        } else {
+            Vec::new()
+        };
+
+        // Render and write
+        if self.split_by_module {
+            write_split(
+                &self.graph,
+                &*self.renderer,
+                &self.output_dir,
+                &self.format_ext,
+            )?;
+        } else {
+            let rendered = self.renderer.render(self.graph.as_graph())?;
+            write_to_output(rendered, &self.output_dir)?;
+        }
+
+        // Report violations (after writing so the diagram is still produced)
+        for v in &violations {
+            eprintln!("RULE VIOLATION: {}", v.message());
+        }
+
+        Ok(())
+    }
+
+    pub fn check_violations_only(&self) -> Vec<String> {
+        if self.allow_rules.is_empty() && self.deny_rules.is_empty() {
+            return Vec::new();
+        }
+        check_boundary_rules(self.graph.as_graph(), &self.allow_rules, &self.deny_rules)
+            .into_iter()
+            .map(|v| v.message())
+            .collect()
+    }
+}
+
+pub fn write_split(
+    graph: &NormalizedGraph,
+    renderer: &dyn DiagramRenderer,
+    output_dir: &Option<PathBuf>,
+    ext: &str,
+) -> Result<(), DomainError> {
+    let dir = output_dir
+        .clone()
+        .unwrap_or_else(|| PathBuf::from("."));
+
+    let overview = renderer.render(graph.as_graph())?;
+    let overview_file = RenderedFile::new(
+        &format!("overview.{ext}"),
+        overview.files().first().map(|f| f.content()).unwrap_or(""),
+    )?;
+    write_file_to_dir(&dir, overview_file)?;
+
+    for module in graph.modules() {
+        let subgraph = graph.subgraph_by_module(&module);
+        let cross_deps = graph.cross_module_deps_for(&module);
+        let module_output = renderer.render_for_module(&subgraph, &module, &cross_deps)?;
+        let module_file = RenderedFile::new(
+            &format!("{}.{ext}", module.as_str().to_lowercase()),
+            module_output.files().first().map(|f| f.content()).unwrap_or(""),
+        )?;
+        write_file_to_dir(&dir, module_file)?;
+    }
+
+    Ok(())
+}
+
+fn write_file_to_dir(dir: &PathBuf, file: RenderedFile) -> Result<(), DomainError> {
+    let path = dir.join(file.name());
+    std::fs::create_dir_all(dir)
+        .map_err(|e| DomainError::IoError(e.to_string()))?;
+    std::fs::write(&path, file.content())
+        .map_err(|e| DomainError::IoError(e.to_string()))?;
+    Ok(())
+}
+
+fn write_to_output(rendered: RenderOutput, output: &Option<PathBuf>) -> Result<(), DomainError> {
+    let content = rendered.files().first().map(|f| f.content()).unwrap_or("");
+    match output {
+        Some(path) => {
+            if let Some(parent) = path.parent() {
+                std::fs::create_dir_all(parent)
+                    .map_err(|e| DomainError::IoError(e.to_string()))?;
+            }
+            std::fs::write(path, content)
+                .map_err(|e| DomainError::IoError(e.to_string()))
+        }
+        None => {
+            print!("{content}");
+            Ok(())
+        }
+    }
+}

crates/application/src/use_cases/mod.rs

@@ -0,0 +1,3 @@
+pub mod check_freshness;
+pub mod diff_diagram;
+pub mod generate_diagram;