Task: Generate a Multi-Directory C++ Project Makefile

You are building a top-level Makefile for a C++ project with three sub-components:

engine/
  src/   (produces libengine.a)
renderer/
  src/   (produces librenderer.a, depends on libengine.a)
app/
  src/   (produces bin/game, links libengine.a and librenderer.a)

Top-level Makefile requirements:

• Use $(MAKE) for all recursive calls into sub-directories, never bare make
• Build order: engine first, then renderer, then app (explicit dependency ordering)
• all builds everything in correct order
• clean calls $(MAKE) clean in each sub-directory
• test calls $(MAKE) test in each sub-directory
• help target present with ## comments

For the app/ component Makefile (produce this as app/Makefile):

• Compiler: CXX ?= g++
• Source glob: $(wildcard src/*.cpp)
• Object files in build/ directory using order-only prerequisite | $(BUILDDIR)
• Dependency tracking: -MMD -MP and -include of generated .d files
• Modern GNU Make header

Produce both the top-level Makefile and app/Makefile.