Sign APK in classic SignApk format so it installs on Android <= 4.2

Replace jarsigner + zipalign.py with tools/signapk.py, which writes the v1
signature the way old JarUtils expects and aligns the APK.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 # Builds Minamp.apk for Android 2.2+ without the Android SDK/Gradle:
-# aapt2 (resources) -> javac (Java 8 bytecode vs android-8 API) -> dx (dex 035) -> jarsigner (v1/SHA-1) -> zipalign.
+# aapt2 (resources) -> javac (Java 8 bytecode vs android-8 API) -> dx (dex 035) -> signapk.py (v1/SHA-1 + zipalign).
 set -euo pipefail
 cd "$(dirname "$0")"
 T=${TOOLS:-$PWD/toolchain}
@@ -28,8 +28,6 @@ java -Xmx2g -jar $T/dx.jar --dex --output=$OUT/classes.dex $OUT/classes 2>&1 | g
 echo "== package"
 cp $OUT/base.apk $OUT/unsigned.apk
 (cd $OUT && zip -q unsigned.apk classes.dex)
-jarsigner -keystore ${KEYSTORE:-$T/minamp.keystore} -storepass minamp123 -keypass minamp123 \
-  -digestalg SHA-1 -sigalg SHA1withRSA -signedjar $OUT/signed.apk $OUT/unsigned.apk minamp 2>&1 \
-  | grep -v -E '^Picked up|^$|Warning|risk|self-signed|POSIX|ignored when signing|not protected' || true
-python3 tools/zipalign.py $OUT/signed.apk $OUT/Minamp.apk
+# v1 signature in the classic Android SignApk format (jarsigner's modern output won't install on Android <= 4.2)
+python3 tools/signapk.py ${KEYSTORE:-$T/minamp.keystore} ${KEYPASS:-minamp123} $OUT/unsigned.apk $OUT/Minamp.apk
 ls -la $OUT/Minamp.apk