Makefile.android now downloads fftw-3.3.11.tar.gz automatically instead
of requiring the user to place sources in third_party/fftw/ manually.
The FFTW_SRC variable can still be overridden to use an existing copy.
Added distclean target. Updated README accordingly.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>