refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/drivers/driver_asynq_cron/scheduler_test.go

@@ -0,0 +1,42 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_asynq_cron
+
+import "testing"
+
+func TestWaitForStop(t *testing.T) {
+	tests := []struct {
+		name        string
+		closeDone   bool
+		closeSignal bool
+		wantSignal  bool
+	}{
+		{
+			name:      "explicit stop",
+			closeDone: true,
+		},
+		{
+			name:        "process signal",
+			closeSignal: true,
+			wantSignal:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			done := make(chan struct{})
+			signals := make(chan struct{})
+			if tt.closeDone {
+				close(done)
+			}
+			if tt.closeSignal {
+				close(signals)
+			}
+
+			if got := waitForStop(done, signals); got != tt.wantSignal {
+				t.Errorf("waitForStop() = %t, want %t", got, tt.wantSignal)
+			}
+		})
+	}
+}