test(artifact): document serial test constraints

Most artifact tests now run in parallel, so the few intentional exceptions are easy to mistake for omissions. Record the process-global state each relies on to prevent future cleanup from making them unsafe or flaky.

Author: wesm

internal/artifact/export_test.go

@@ -1073,6 +1073,7 @@ func TestLatestValidCheckpointFallsBackPastSemanticCandidateAndDefersFuture(t *t
 }
 
 func TestExportSpoolConstructionJoinsCleanupFailures(t *testing.T) {
+	// Serial: overrides package-level spool filesystem hooks.
 	chmodFailure := errors.New("injected spool setup failure")
 	cleanupFailure := errors.New("injected spool cleanup failure")
 	previousChmod := exportSpoolChmod

internal/artifact/import_checkpoint_test.go

@@ -515,6 +515,7 @@ func TestDecodeImportCheckpointBoundsTopLevelFieldState(t *testing.T) {
 }
 
 func TestPreflightImportCheckpointVersionBoundsAllocations(t *testing.T) {
+	// Serial: AllocsPerRun measures process-global allocation state.
 	const sessionCount = 1_000
 	var body strings.Builder
 	fmt.Fprintf(&body, `{"origin":%q,"seq":7,"sessions":{`, contractOrigin)
@@ -544,6 +545,7 @@ func TestPreflightImportCheckpointVersionBoundsAllocations(t *testing.T) {
 func TestDecodeImportCheckpointRejectsCurrentExtraFieldBeforeItsValue(
 	t *testing.T,
 ) {
+	// Serial: AllocsPerRun measures process-global allocation state.
 	var body strings.Builder
 	body.WriteString(`{"v":1,"extra":{`)
 	for i := range 10_000 {

internal/artifact/repository_test.go

@@ -101,6 +101,7 @@ func TestOpenRepositoryFollowsFinalRootSymlink(t *testing.T) {
 }
 
 func TestOpenRepositoryRetainsAbsoluteCanonicalRoot(t *testing.T) {
+	// Serial: Chdir changes the process-wide working directory.
 	dataDir := t.TempDir()
 	// Chdir to the temp dir's parent so the relative path stays on one
 	// volume; on Windows CI the checkout and temp dirs are on different

internal/artifact/wire_codec_test.go

@@ -651,6 +651,7 @@ func TestWireCodecStreamsMultiMegabyteArtifactsWithBoundedBuffers(t *testing.T)
 }
 
 func TestWireCodecAllocatedBytesStayBoundedAsArtifactsGrow(t *testing.T) {
+	// Serial: Benchmark reads process-global allocation statistics.
 	const (
 		smallSize = int64(32 << 10)
 		largeSize = int64(12 << 20)