docs: rewrite README with comprehensive toolchain usage guide

Dockerfile.yml

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+# Native linux/arm64 Android build toolchain for CI.
+# Builds release artifacts only — no emulator, no test/system images.
+# eclipse-temurin publishes a native linux/arm64 manifest, so this runs
+# without emulation on an OCI Ampere (aarch64) runner.
+FROM eclipse-temurin:21-jdk-noble
+
+# --- Version pins (bump deliberately; this is what your app repos trust) ---
+# cmdline-tools: find the current build number at
+#   https://developer.android.com/studio#command-line-tools-only
+ARG CMDLINE_TOOLS_VERSION=13114758
+ARG BUILD_TOOLS_VERSION=35.0.0
+ARG PLATFORM_VERSION=android-35
+
+ENV ANDROID_SDK_ROOT=/opt/android-sdk \
+    ANDROID_HOME=/opt/android-sdk \
+    DEBIAN_FRONTEND=noninteractive
+
+# git is needed by some Gradle plugins; unzip/curl for SDK install.
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        curl unzip git ca-certificates && \
+    rm -rf /var/lib/apt/lists/*
+
+# Install the command-line tools into the canonical "latest" location.
+RUN mkdir -p ${ANDROID_SDK_ROOT}/cmdline-tools && \
+    curl -fsSL -o /tmp/tools.zip \
+      "https://dl.google.com/android/repository/commandlinetools-linux-${CMDLINE_TOOLS_VERSION}_latest.zip" && \
+    unzip -q /tmp/tools.zip -d ${ANDROID_SDK_ROOT}/cmdline-tools && \
+    mv ${ANDROID_SDK_ROOT}/cmdline-tools/cmdline-tools ${ANDROID_SDK_ROOT}/cmdline-tools/latest && \
+    rm /tmp/tools.zip
+
+ENV PATH=${PATH}:${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin:${ANDROID_SDK_ROOT}/platform-tools
+
+# Accept licenses and bake the SDK packages into the image so prod builds
+# don't depend on Google's endpoint at job time.
+# aapt2 and the build-tools binaries ship native arm64 for 35.x, so APK/AAB
+# packaging runs natively on aarch64.
+RUN yes | sdkmanager --licenses >/dev/null && \
+    sdkmanager --install \
+      "platform-tools" \
+      "platforms;${PLATFORM_VERSION}" \
+      "build-tools;${BUILD_TOOLS_VERSION}" && \
+    rm -rf ${ANDROID_SDK_ROOT}/.android
+
+# Sanity: fail the image build if the toolchain isn't actually usable.
+RUN java -version && sdkmanager --version && \
+    test -d "${ANDROID_SDK_ROOT}/platforms/${PLATFORM_VERSION}"
+
+WORKDIR /workspace