fix: remove nil-optional superMu pattern, require in all FlusherConfigs

superMu is mandatory for correctness — all superblock mutation+persist
must be serialized. Remove the nil guard in updateSuperblockCheckpoint
and add SuperMu to all 7 test FlusherConfig sites.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

weed/storage/blockvol/blockvol_qa_test.go

@@ -1495,6 +1495,7 @@ func testQAFlushCheckpointPersists(t *testing.T) {
 	f := NewFlusher(FlusherConfig{
 		FD:       v.fd,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour,
@@ -1586,6 +1587,7 @@ func testQAFlushWALReclaimThenWrite(t *testing.T) {
 	f := NewFlusher(FlusherConfig{
 		FD:       v.fd,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour,
@@ -1849,6 +1851,7 @@ func testQARecoverAfterFlushThenCrash(t *testing.T) {
 
 	f := NewFlusher(FlusherConfig{
 		FD: v.fd, Super: &v.super, WAL: v.wal, DirtyMap: v.dirtyMap,
+		SuperMu:  &v.superMu,
 		Interval: 1 * time.Hour,
 	})
 	if err := f.FlushOnce(); err != nil {
@@ -3413,6 +3416,7 @@ func testQAFlushPartialWALWrap(t *testing.T) {
 	f := NewFlusher(FlusherConfig{
 		FD:       v.fd,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour, // manual only

weed/storage/blockvol/flusher.go

@@ -420,10 +420,8 @@ func (f *Flusher) flushOnceLocked() error {
 // updateSuperblockCheckpoint writes the updated checkpoint to disk.
 // Acquires superMu to serialize against syncWithWALProgress (group commit).
 func (f *Flusher) updateSuperblockCheckpoint(checkpointLSN uint64, walTail uint64) error {
-	if f.superMu != nil {
-		f.superMu.Lock()
-		defer f.superMu.Unlock()
-	}
+	f.superMu.Lock()
+	defer f.superMu.Unlock()
 
 	f.super.WALCheckpointLSN = checkpointLSN
 	f.super.WALHead = f.wal.LogicalHead()

weed/storage/blockvol/flusher_test.go

@@ -48,6 +48,7 @@ func createTestVolWithFlusher(t *testing.T) (*BlockVol, *Flusher) {
 	f := NewFlusher(FlusherConfig{
 		FD:       v.fd,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour, // don't auto-flush in tests
@@ -343,6 +344,7 @@ func testFlusherErrorLogged(t *testing.T) {
 	f := NewFlusher(FlusherConfig{
 		FD:       closedFD,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour,

weed/storage/blockvol/recovery_test.go

@@ -230,6 +230,7 @@ func testRecoverAfterCheckpoint(t *testing.T) {
 	f := NewFlusher(FlusherConfig{
 		FD:       v.fd,
 		Super:    &v.super,
+		SuperMu:  &v.superMu,
 		WAL:      v.wal,
 		DirtyMap: v.dirtyMap,
 		Interval: 1 * time.Hour,