iceberg: create missing stage-create marker directories exclusively

The marker path starts at the table bucket directory, which carries the bucket
record; a plain create after a missed lookup replaces a directory another writer
made in between, with whatever it carries.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

weed/s3api/iceberg/iceberg_create_table_conflict_test.go

@@ -960,6 +960,26 @@ func TestSaveMetadataBlobKeepsATableDirectoryCreatedSinceTheLookup(t *testing.T)
 	}
 }
 
+// A stage-create marker write keeps the attributes of a directory on its path
+// that another writer created after the marker's lookup missed it.
+func TestStageCreateMarkerKeepsADirectoryCreatedSinceTheLookup(t *testing.T) {
+	e := newConflictEnv(t)
+	bucketDir := path.Join(s3tables.TablesPath, conflictBucket)
+	attrs := map[string][]byte{"probe": []byte("kept")}
+	e.hooks.beforeCreate = func(req *filer_pb.CreateEntryRequest) {
+		if req.Directory == bucketDir && req.Entry.Name == stageCreateMarkerDirName {
+			e.hooks.beforeCreate = nil
+			e.filer.Put(bucketDir, stageCreateMarkerDirName, attrs)
+		}
+	}
+	if err := e.server.writeStageCreateMarker(t.Context(), conflictBucket, []string{conflictNamespace}, conflictTable, uuid.New(), conflictLocation, ""); err != nil {
+		t.Fatalf("write stage-create marker: %v", err)
+	}
+	if got := e.filer.Get(bucketDir, stageCreateMarkerDirName); got == nil || string(got.Extended["probe"]) != "kept" {
+		t.Errorf("directory replaced by the marker's directory create: %+v", got)
+	}
+}
+
 // Two stage-create commits for one name: one creates the table and the other is
 // told it lost, and the file the catalog points at belongs to the winner.
 func TestConcurrentFinalizeCreateOnCommitHasOneWinner(t *testing.T) {

weed/s3api/iceberg/stage_create.go

@@ -191,6 +191,7 @@ func (s *Server) writeStageCreateMarker(ctx context.Context, bucketName string,
 
 			resp, createErr := client.CreateEntry(opCtx, &filer_pb.CreateEntryRequest{
 				Directory: parent,
+				OExcl:     true,
 				Entry: &filer_pb.Entry{
 					Name:        name,
 					IsDirectory: true,