test(build): close stylesheet scanner mutants

Prove sorted, deduplicated CSS/SCSS class extraction across starts, compounds, delimiters, invalid candidates, and invalid stylesheet module names; simplify token scanning to one boundary loop and classify only deterministic non-terminating progress mutants.

req: style/001

req: diagnostics/004

req: test/021

hemx-build/src/lib.rs

@@ -1451,7 +1451,7 @@ fn stylesheet_class_tokens(source: &str) -> Vec<&str> {
             i += 1;
             continue;
         }
-        let mut end = start + 1;
+        let mut end = start;
         while end < bytes.len() && is_class_continue(bytes[end]) {
             end += 1;
         }
@@ -2983,6 +2983,37 @@ mod tests {
         let _ = std::fs::remove_dir_all(&base);
     }
 
+    #[test]
+    fn stylesheet_class_scanner_is_sorted_deduplicated_and_boundary_aware() {
+        assert_eq!(stylesheet_class_tokens(""), Vec::<&str>::new());
+        assert_eq!(stylesheet_class_tokens(".a"), vec!["a"]);
+        assert_eq!(stylesheet_class_tokens(".ab"), vec!["ab"]);
+        assert_eq!(stylesheet_class_tokens("x.skip .kept"), vec!["kept"]);
+        assert_eq!(stylesheet_class_tokens("x..kept"), vec!["kept"]);
+        assert_eq!(stylesheet_class_tokens("..kept"), vec!["kept"]);
+        assert_eq!(stylesheet_class_tokens(". .kept"), vec!["kept"]);
+        assert_eq!(stylesheet_class_tokens(".1 .kept"), vec!["kept"]);
+        assert_eq!(stylesheet_class_tokens(".é .kept"), vec!["kept"]);
+        assert!(!preceded_by_class_in_compound(b"tag.", 3));
+        assert!(preceded_by_class_in_compound(b".first.", 6));
+        assert_eq!(
+            stylesheet_class_tokens(
+                ".z9, .alpha.alpha, ._private, .-prefixed, .compound.second, tag.skipped, name-.skipped, name_.skipped"
+            ),
+            vec!["-prefixed", "_private", "alpha", "compound", "second", "z9"]
+        );
+        assert_eq!(
+            stylesheet_class_tokens(
+                " .space\n.newline\r.return\t.tab,.comma{.open}.close>.child+.adjacent~.sibling(.paren)"
+            ),
+            vec![
+                "adjacent", "child", "close", "comma", "newline", "open", "paren", "return",
+                "sibling", "space", "tab"
+            ]
+        );
+        // test req: style/001
+    }
+
     #[test]
     fn class_and_form_resource_contracts_fail_closed_and_deduplicate() {
         let base = test_dir("hemx-build-resource-collisions");
@@ -2990,6 +3021,20 @@ mod tests {
         let out = base.join("out");
         let _ = std::fs::remove_dir_all(&base);
         std::fs::create_dir_all(&templates).unwrap();
+        let invalid_stylesheet = templates.join("123.css");
+        std::fs::write(&invalid_stylesheet, ".valid {}").unwrap();
+        let invalid_stylesheet_name = app()
+            .template_dir(&templates)
+            .out_dir(&out)
+            .run()
+            .unwrap_err();
+        assert_eq!(invalid_stylesheet_name.kind(), io::ErrorKind::InvalidData);
+        assert_eq!(
+            invalid_stylesheet_name.to_string(),
+            "invalid template name `123`; expected a Rust module identifier"
+        );
+        std::fs::remove_file(invalid_stylesheet).unwrap();
+
         let invalid_class_template = templates.join("invalid_class.heml");
         std::fs::write(&invalid_class_template, r#"<div class="123"></div>"#).unwrap();
         let invalid_class = app()