feat: add Android cross-compile script for phone benchmarking

Add build_android.sh that cross-compiles the C secp256k1 library for
ARM64 using the Android NDK and places the .so in benchmark/src/main/
jniLibs/ where the benchmark APK will package it.

Usage:
  cd quartz/src/main/c/secp256k1
  ./build_android.sh
  cd ../../../../..
  ./gradlew :benchmark:connectedAndroidTest

Also:
- Fix Secp256k1InstanceC.android.kt to use Secp256k1C JNI binding
  class (same pattern as JVM) so the JNI method names match
  Java_com_vitorpamplona_quartz_utils_Secp256k1C_native*
- Add benchmark/src/main/jniLibs/ to .gitignore

https://claude.ai/code/session_011KVZhDcV2G7idNWEBz12GY

quartz/src/main/c/secp256k1/CMakeLists.txt

@@ -39,7 +39,15 @@ target_include_directories(secp256k1_amethyst_jni PUBLIC ${CMAKE_CURRENT_SOURCE_
 
 # ==================== JNI bridge ====================
 
-if(JNI_INCLUDE_DIR)
+if(ANDROID)
+    # Android NDK: JNI headers are always available, build shared lib with JNI bridge
+    target_sources(secp256k1_amethyst_jni PRIVATE jni_bridge.c)
+    find_library(log-lib log)
+    if(log-lib)
+        target_link_libraries(secp256k1_amethyst_jni ${log-lib})
+    endif()
+elseif(JNI_INCLUDE_DIR)
+    # Desktop JVM: JNI headers provided externally
     target_sources(secp256k1_amethyst_jni PRIVATE jni_bridge.c)
     target_include_directories(secp256k1_amethyst_jni PRIVATE ${JNI_INCLUDE_DIR})
     if(JNI_INCLUDE_DIR_PLATFORM)
@@ -47,17 +55,6 @@ if(JNI_INCLUDE_DIR)
     endif()
 endif()
 
-# ==================== Android NDK build ====================
-
-if(ANDROID)
-    # Android-specific: build shared library for JNI
-    find_library(log-lib log)
-    target_link_libraries(secp256k1_amethyst_jni ${log-lib})
-
-    # Include JNI headers from NDK
-    target_sources(secp256k1_amethyst_jni PRIVATE jni_bridge.c)
-endif()
-
 # ==================== Standalone benchmark ====================
 
 if(NOT ANDROID)

quartz/src/main/c/secp256k1/build_android.sh

@@ -0,0 +1,78 @@
+#!/bin/bash
+#
+# Cross-compile secp256k1 C library for Android ARM64 and package
+# into the benchmark APK's jniLibs directory.
+#
+# Run from your development machine (macOS or Linux) with Android NDK installed.
+#
+# Usage:
+#   cd quartz/src/main/c/secp256k1
+#   ./build_android.sh
+#   # Then run the benchmark:
+#   cd ../../../../..
+#   ./gradlew :benchmark:connectedAndroidTest -Pandroid.testInstrumentationRunnerArguments.class=com.vitorpamplona.quartz.benchmark.Secp256k1CBenchmark
+#
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+PROJECT_ROOT="$SCRIPT_DIR/../../../../.."
+JNILIB_DIR="$PROJECT_ROOT/benchmark/src/main/jniLibs"
+
+# Find NDK
+NDK="${ANDROID_NDK_HOME:-${ANDROID_HOME:-$HOME/Library/Android/sdk}/ndk/$(ls ${ANDROID_HOME:-$HOME/Library/Android/sdk}/ndk/ 2>/dev/null | sort -V | tail -1)}"
+if [ ! -d "$NDK" ]; then
+    echo "ERROR: Android NDK not found."
+    echo "Set ANDROID_NDK_HOME or install NDK via: sdkmanager 'ndk;27.2.12479018'"
+    exit 1
+fi
+
+# Find clang for ARM64
+HOST_TAG="linux-x86_64"
+if [[ "$(uname)" == "Darwin" ]]; then HOST_TAG="darwin-x86_64"; fi
+CC="$NDK/toolchains/llvm/prebuilt/$HOST_TAG/bin/aarch64-linux-android26-clang"
+CC_X86="$NDK/toolchains/llvm/prebuilt/$HOST_TAG/bin/x86_64-linux-android26-clang"
+
+if [ ! -f "$CC" ]; then
+    echo "ERROR: ARM64 clang not found at: $CC"
+    echo "NDK path: $NDK"
+    exit 1
+fi
+
+echo "NDK: $NDK"
+echo "Output: $JNILIB_DIR"
+echo ""
+
+SOURCES="field.c scalar.c point.c schnorr.c sha256.c jni_bridge.c"
+
+# Build ARM64 shared library
+echo "Building arm64-v8a..."
+mkdir -p "$JNILIB_DIR/arm64-v8a"
+$CC -O3 -march=armv8-a+crypto -fomit-frame-pointer \
+    -shared -fPIC \
+    -I"$SCRIPT_DIR" \
+    $(cd "$SCRIPT_DIR" && echo $SOURCES) \
+    -o "$JNILIB_DIR/arm64-v8a/libsecp256k1_amethyst_jni.so" \
+    -lm
+echo "  → $(wc -c < "$JNILIB_DIR/arm64-v8a/libsecp256k1_amethyst_jni.so") bytes"
+
+# Build x86_64 shared library (for emulator)
+if [ -f "$CC_X86" ]; then
+    echo "Building x86_64..."
+    mkdir -p "$JNILIB_DIR/x86_64"
+    $CC_X86 -O3 -march=x86-64-v2 -mbmi2 -msha -msse4.1 -fomit-frame-pointer \
+        -shared -fPIC \
+        -I"$SCRIPT_DIR" \
+        $(cd "$SCRIPT_DIR" && echo $SOURCES) \
+        -o "$JNILIB_DIR/x86_64/libsecp256k1_amethyst_jni.so" \
+        -lm
+    echo "  → $(wc -c < "$JNILIB_DIR/x86_64/libsecp256k1_amethyst_jni.so") bytes"
+fi
+
+echo ""
+echo "Done! Native libraries placed in:"
+echo "  $JNILIB_DIR/"
+echo ""
+echo "Now run the benchmark:"
+echo "  ./gradlew :benchmark:connectedAndroidTest \\"
+echo "    -Pandroid.testInstrumentationRunnerArguments.class=com.vitorpamplona.quartz.benchmark.Secp256k1CBenchmark"