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/pkg/util/goroutine_test.go

@@ -0,0 +1,38 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestGoRecoversPanic(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(1)
+
+	// Go should swallow the panic without crashing the test process
+	Go(func() {
+		defer wg.Done()
+		panic("boom")
+	})
+
+	wg.Wait()
+}
+
+func TestGoRunsNormally(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(1)
+	ran := false
+
+	Go(func() {
+		defer wg.Done()
+		ran = true
+	})
+
+	wg.Wait()
+	if !ran {
+		t.Fatal("expected fn to run")
+	}
+}