refactor(structure): group platform, infra, and shared packages

Reorganize internal packages into platform/infra/shared layers and update
imports, docs, and seed-count tests to match current system configs.

internal/infra/persistence/batchwriter/errs.go

@@ -0,0 +1,8 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package batchwriter
+
+import "errors"
+
+var errNilFlushFunc = errors.New("batchwriter: flush func is required")