GNU/Linux static-ish build

Prefer a mostly static binary when distro matrix is large:

cmake -S . -B build-static -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_EXE_LINKER_FLAGS="-static-libgcc"
cmake --build build-static -j

Full static SDL2 is painful; document dynamic SDL2 .so + rpath as interim. Ship AppImage later if needed.