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/cache/ram/cache_test.go

@@ -0,0 +1,38 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package ram
+
+import "testing"
+
+func TestCacheSetGetInvalidate(t *testing.T) {
+	cache := MustNew[string, int](Options{MaximumSize: 8})
+
+	cache.Set("count", 3)
+
+	got, ok := cache.GetIfPresent("count")
+	if !ok {
+		t.Fatal("GetIfPresent(count) ok = false, want true")
+	}
+	if got != 3 {
+		t.Fatalf("GetIfPresent(count) = %d, want %d", got, 3)
+	}
+
+	cache.Invalidate("count")
+	if _, ok := cache.GetIfPresent("count"); ok {
+		t.Fatal("GetIfPresent(count) after Invalidate ok = true, want false")
+	}
+}
+
+func TestCacheInvalidateAll(t *testing.T) {
+	cache := MustNew[string, string](Options{MaximumSize: 8})
+
+	cache.Set("a", "1")
+	cache.Set("b", "2")
+
+	cache.InvalidateAll()
+
+	if cache.EstimatedSize() != 0 {
+		t.Fatalf("EstimatedSize() after InvalidateAll = %d, want 0", cache.EstimatedSize())
+	}
+}