build(android): add --device/--abi to build only the needed architecture

An on-device test build compiled all four ABIs (arm64/arm/x86/x86_64),
so three of the four Rust compiles were thrown away. That dominated the
build time when iterating against a connected phone.

--device resolves the attached device's ABI via adb and targets just
that triple; --abi <target> selects one explicitly; ABI= works as an
env var. Default behaviour is unchanged (all four), since a
distributable universal APK genuinely needs them.

  bun run android:build:device
  bun run android:build:release:device

scripts/build-android.sh

@@ -18,15 +18,50 @@ echo ""
 # Parse args: build type (debug/release) and optional --clean flag.
 # By default the build is INCREMENTAL — Cargo and Vite reuse their caches.
 # Pass --clean (or CLEAN=1) to wipe all caches for a from-scratch build.
+#
+# ABI selection: by default Tauri builds all four ABIs (arm64/arm/x86/x86_64),
+# which is what a distributable universal APK needs — but for an on-device test
+# it means three wasted Rust compiles. Pass --device (or ABI=aarch64) to build
+# only the connected device's architecture; --abi <t> targets one explicitly.
 BUILD_TYPE="debug"
 CLEAN="${CLEAN:-0}"
+ABI="${ABI:-}"
+next_is_abi=0
 for arg in "$@"; do
+    if [ "$next_is_abi" = "1" ]; then
+        ABI="$arg"
+        next_is_abi=0
+        continue
+    fi
     case "$arg" in
         --clean) CLEAN=1 ;;
+        --abi) next_is_abi=1 ;;
+        --device) ABI="device" ;;
         debug|release) BUILD_TYPE="$arg" ;;
     esac
 done
 
+# Resolve --device to the attached device's Rust target triple.
+if [ "$ABI" = "device" ]; then
+    device_abi="$(adb shell getprop ro.product.cpu.abi 2>/dev/null | tr -d '\r\n')"
+    case "$device_abi" in
+        arm64-v8a)   ABI="aarch64" ;;
+        armeabi-v7a) ABI="armv7" ;;
+        x86_64)      ABI="x86_64" ;;
+        x86)         ABI="i686" ;;
+        *)
+            echo "⚠️  Could not detect device ABI (got '${device_abi:-none}') — building all targets."
+            ABI=""
+            ;;
+    esac
+    [ -n "$ABI" ] && echo "🎯 Device ABI $device_abi → building only '$ABI'"
+fi
+
+TARGET_ARGS=()
+if [ -n "$ABI" ]; then
+    TARGET_ARGS=(--target "$ABI")
+fi
+
 # Step 0: Optionally clear build caches for a fully fresh build.
 if [ "$CLEAN" = "1" ]; then
     echo "🧹 Clearing build caches (clean build)..."
@@ -48,10 +83,10 @@ if [ "$BUILD_TYPE" = "release" ]; then
     # after sync-android-sources.sh, since gen/android is (re)generated there.
     ./scripts/write-keystore-properties.sh
     echo "📦 Building release APK..."
-    bun run tauri android build --apk true
+    bun run tauri android build --apk true "${TARGET_ARGS[@]}"
 else
     echo "📦 Building debug APK..."
-    bun run tauri android build --apk true --debug
+    bun run tauri android build --apk true --debug "${TARGET_ARGS[@]}"
 fi
 
 echo ""