test(build): prove stale artifact refresh

Drive AppBuilder through a changed template and prove generated Rust and symbols replace stale handles. Tighten the internal write helper to return only the result its callers use and classify the filesystem-equivalent read-guard mutant.

req: build/009

req: test/021

hemx-build/src/lib.rs

@@ -236,15 +236,14 @@ impl AppBuilder {
     }
 }
 
-fn write_if_changed(path: &Path, contents: &[u8]) -> io::Result<bool> {
+fn write_if_changed(path: &Path, contents: &[u8]) -> io::Result<()> {
     match std::fs::read(path) {
-        Ok(existing) if existing == contents => return Ok(false),
+        Ok(existing) if existing == contents => return Ok(()),
         Ok(_) => {}
         Err(error) if error.kind() == io::ErrorKind::NotFound => {}
         Err(error) => return Err(error),
     }
-    std::fs::write(path, contents)?;
-    Ok(true)
+    std::fs::write(path, contents)
 }
 
 pub fn app() -> AppBuilder {
@@ -2593,6 +2592,45 @@ mod tests {
         let _ = std::fs::remove_dir_all(&base);
     }
 
+    #[test]
+    fn changed_template_refreshes_generated_artifacts() {
+        let base = test_dir("hemx-build-refresh");
+        let templates = base.join("templates");
+        let out = base.join("out");
+        let template = templates.join("panel.heml");
+        let _ = std::fs::remove_dir_all(&base);
+        std::fs::create_dir_all(&templates).unwrap();
+        std::fs::write(
+            &template,
+            r#"<button data-hemx-handle="save">Save</button>"#,
+        )
+        .unwrap();
+
+        app().template_dir(&templates).out_dir(&out).run().unwrap();
+        let initial_generated = std::fs::read_to_string(out.join("hemx.generated.rs")).unwrap();
+        let initial_symbols = std::fs::read_to_string(out.join("hemx.syms")).unwrap();
+        assert!(initial_generated.contains("pub const save"));
+        assert!(initial_symbols.contains("save"));
+
+        std::fs::write(
+            &template,
+            r#"<button data-hemx-handle="cancel">Cancel</button>"#,
+        )
+        .unwrap();
+        app().template_dir(&templates).out_dir(&out).run().unwrap();
+
+        let refreshed_generated = std::fs::read_to_string(out.join("hemx.generated.rs")).unwrap();
+        let refreshed_symbols = std::fs::read_to_string(out.join("hemx.syms")).unwrap();
+        assert_ne!(refreshed_generated, initial_generated);
+        assert_ne!(refreshed_symbols, initial_symbols);
+        assert!(refreshed_generated.contains("pub const cancel"));
+        assert!(!refreshed_generated.contains("pub const save"));
+        assert!(refreshed_symbols.contains("cancel"));
+        assert!(!refreshed_symbols.contains("save"));
+        let _ = std::fs::remove_dir_all(&base);
+        // test req: build/009
+    }
+
     #[test]
     fn emits_generated_resources_from_heml() {
         // req: codegen/003