Makefile.windows builds libwdsp.dll and libwdsp.a for 64-bit Windows
on Linux. FFTW 3.3.11 Windows binaries are downloaded automatically
from fftw.org and import libraries are generated with dlltool.
README updated with Windows build instructions and FFTW version bump.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
