FastLED ships a host-test stub src/platforms/stub/Arduino.h. When another
merged library (e.g. U8g2) does #include <Arduino.h>, the stub resolved
first and broke the build: 'yield' was not declared in U8g2lib.cpp and
fl::string polluted String overload resolution inside the core WString.h.
Reuse the _SHADOW_STD_HEADERS mechanism so the stub's directory stays off
the global include path; FastLED reaches it via a file-relative include.
39b2ccf515