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

.cargo/mutants.toml

@@ -16,6 +16,8 @@
 # - write_if_changed propagates non-NotFound read errors; for ordinary filesystem
 #   paths, attempting the same write returns the same OS error, so the guard mutant
 #   is externally equivalent while create/update/no-op behavior is mutation-proven.
+# - stylesheet_class_tokens loop-progress mutants are deterministically
+#   non-terminating; sorted, deduplicated, boundary-aware outputs are asserted.
 exclude_re = [
   "test_process_try_wait",
   "test_process_poll_delay",
@@ -34,4 +36,6 @@ exclude_re = [
   "replace surface_for_heml_source.* with surface_for_heml_source.*unwrap\\(\\) in AppBuilder::run",
   "replace entry\\? with entry.unwrap\\(\\) in collect_input_files_into",
   "replace match guard error.kind\\(\\) == io::ErrorKind::NotFound with true in write_if_changed",
+  "replace \\+= with (?:-=|\\*=) in stylesheet_class_tokens",
+  "replace 1 with 0 in stylesheet_class_tokens",
 ]