docs: add BUILD.md with step-by-step mobile build guide

packages/mobile/BUILD.md

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+# KirinCode Mobile — Build Guide
+
+## Prerequisites
+
+Your Mac has **Xcode 26.5** already. The following need to be installed (one-time setup):
+
+```bash
+# 1. Java 17 (required for Android builds)
+brew install openjdk@17
+echo 'export PATH="/opt/homebrew/opt/openjdk@17/bin:$PATH"' >> ~/.zshrc
+source ~/.zshrc
+
+# 2. CocoaPods (required for iOS builds)
+brew install cocoapods
+
+# 3. Node.js (required for React Native)
+brew install node
+
+# 4. Android Studio (required for Android builds)
+# Download from: https://developer.android.com/studio
+# After install, add to ~/.zshrc:
+# export ANDROID_HOME=$HOME/Library/Android/sdk
+# export PATH=$ANDROID_HOME/tools:$ANDROID_HOME/platform-tools:$PATH
+
+# 5. DevEco Studio (required for HarmonyOS builds)
+# Download from: https://developer.huawei.com/consumer/en/devstudio/
+```
+
+## Quick Build
+
+```bash
+cd kirincode
+bash build-mobile.sh all
+```
+
+## Platform-Specific
+
+### iOS (.ipa)
+```bash
+cd packages/mobile
+npm install
+npx expo prebuild --platform ios
+cd ios && pod install
+cd ..
+npx expo run:ios --configuration Release
+# Output: ios/build/
+```
+
+### Android (.apk)
+```bash
+cd packages/mobile
+npm install
+npx expo prebuild --platform android
+cd android && ./gradlew assembleRelease
+# Output: app/build/outputs/apk/release/
+```
+
+### HarmonyOS (.hap)
+1. Open DevEco Studio
+2. File → Open → `packages/kirincode-ohos`
+3. Build → Build Hap(s)
+4. Output: `entry/build/default/outputs/default/entry-default-signed.hap`
+
+## Installer Locations (after build)
+
+```
+iOS:        packages/mobile/ios/build/KirinCode.ipa
+Android:    packages/mobile/android/app/build/outputs/apk/release/app-release.apk
+HarmonyOS:  packages/kirincode-ohos/entry/build/default/outputs/default/entry-default-signed.hap
+```