Auto-download FFTW source for Android build

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>

README.md

@@ -17,7 +17,7 @@ wdsp/
 ├── java/
 │   └── org/openhpsdr/dsp/Wdsp.java
 ├── third_party/
-│   ├── fftw/              — FFTW source (download manually, Android only)
+│   ├── fftw-3.3.11/       — FFTW source (downloaded automatically)
 │   ├── fftw-win64/        — FFTW Windows binaries (downloaded automatically)
 │   ├── rnnoise/           — RNNoise noise suppression
 │   └── libspecbleach/     — spectral noise reduction
@@ -169,15 +169,9 @@ make -f Makefile.windows distclean   # + удалить скачанный FFTW
 ### Prerequisites
 
 1. **Android NDK** r23 or newer
-2. **FFTW source** — download and extract into `third_party/fftw/`:
+2. **Java compiler** — for building the `.class` file (Android Studio's JBR or any JDK)
 
-```bash
-wget https://www.fftw.org/fftw-3.3.11.tar.gz
-tar xf fftw-3.3.11.tar.gz
-mv fftw-3.3.11 third_party/fftw
-```
-
-3. **Java compiler** — for building the `.class` file (Android Studio's JBR or any JDK)
+FFTW 3.3.11 скачивается и собирается автоматически. Чтобы использовать уже скачанный исходник, передайте `FFTW_SRC=/path/to/fftw-3.3.11`.
 
 ### Build
 
@@ -227,8 +221,8 @@ make -f Makefile.android \
 ### Clean
 
 ```bash
-make -f Makefile.android clean
-# removes obj/android/ and lib/android/
+make -f Makefile.android clean       # удаляет obj/android/ и lib/android/
+make -f Makefile.android distclean   # + удаляет скачанный FFTW
 ```
 
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