iris-android-app: clear Gradle's native-libs cache, so an ABI switch takes

`build-apk.sh` already removed `app/src/main/jniLibs` before each build,
with its own comment saying why. It does not reach Gradle's own copy:
`mergeReleaseNativeLibs` is up to date against its cached inputs, so a
build that switches ABI packages the previous one. An `--abi x86_64`
release APK containing `lib/arm64-v8a/libmain.so` installed fine and
aborted at startup with `Could not get adapter!: NotFound {
active_backends: VULKAN }` under libndk_translation -- which reads
exactly like the phone's own Vulkan problem and is nothing of the kind.
It cost an hour on 2026-09-07 and was written down rather than fixed.

Scoped to the three native-lib directories rather than all of
app/build, so an ABI change costs the native merge and not the whole
Gradle build. Verified on the case that produced it: this checkout held
an x86_64 libmain.so from emulator work, and `./build-apk.sh release
--abi arm64-v8a` produced an APK whose only .so is
lib/arm64-v8a/libmain.so (7,518,840 bytes) -- that APK is
ai-app-bench a012ff9.

docs/RUST.md's queue box keeps its second half open: the 648 MB debug
bench APK still will not install.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

iris/android-app/build-apk.sh

@@ -57,6 +57,18 @@ export ANDROID_NDK_HOME="$NDK_DIR"
 # finds -- a debug x86_64 emulator build left behind made an arm64 "release"
 # 339 MB on 2026-09-06.
 rm -rf app/src/main/jniLibs
+# ...and Gradle's own copy of them, which `rm -rf jniLibs` does not reach.
+# `mergeReleaseNativeLibs` is *up to date* against its cached inputs, so a
+# build that switches ABI packages the previous ABI: an `--abi x86_64`
+# release APK containing `lib/arm64-v8a/libmain.so` installed fine and
+# aborted at startup with `Could not get adapter!: NotFound {
+# active_backends: VULKAN }` under libndk_translation -- which reads
+# exactly like the phone's own Vulkan problem and is nothing of the kind.
+# Scoped to the merge task's directory rather than all of `app/build`, so
+# an ABI change costs the native merge and not the whole Gradle build.
+rm -rf app/build/intermediates/merged_native_libs \
+       app/build/intermediates/stripped_native_libs \
+       app/build/intermediates/merged_jni_libs
 echo "build-apk.sh: cargo ndk -t $ABI build ${BUILD_TYPE:+(${BUILD_TYPE})} --features \"$FEATURES\""
 if [ "$BUILD_TYPE" = "release" ]; then
     cargo ndk -t "$ABI" -P 29 -o app/src/main/jniLibs/ build --release --features "$FEATURES"