build.extra_flags replaces the esp32 core's platform-level flags,
which include CONFIG_IDF_TARGET_ESP32S3 — required by Arduino_GFX
for RGB panel support. compiler.cpp.extra_flags appends safely.
Usage:
mise run compile # default (e32r35t)
BOARD_TARGET=waveshare_s3 mise run compile # Waveshare 4.3"
BOARD_TARGET=e32r35t mise run all # build + upload + monitor
BOARD_TARGET=waveshare_s3 mise run install-libs # install target-specific libs
Each target gets its own FQBN, build flags, library set, and port detection.
