Add HarmonyOS Next platform support for Flutter Stockfish plugin

[Copilot]

This PR adds comprehensive support for HarmonyOS Next (API Level 10+) to the Flutter Stockfish chess engine plugin, enabling developers to use the powerful Stockfish engine across Android, iOS, and HarmonyOS platforms with a unified API.

Key Features Added

🎯 Complete Platform Integration

• Added ohos platform configuration to pubspec.yaml as an FFI plugin
• Robust multi-method platform detection supporting HarmonyOS Next's various identification patterns
• Native library loading using the same .so approach as Android for consistency

🛠️ Native Build System

• Complete CMake build configuration optimized for HarmonyOS architectures
• Architecture-specific optimizations:
  • ARM64-v8a: NEON SIMD instructions, POPCNT, 64-bit optimizations
  • ARMv7-a: Standard ARM optimizations
  • x86_64: POPCNT support for simulators
• Automatic neural network file downloads for chess evaluation

📚 Developer Experience

• Comprehensive setup guide in HARMONYOS.md covering prerequisites, SDK setup, and troubleshooting
• Complete example HarmonyOS app with EntryAbility and Flutter integration points
• Updated main README with platform support matrix and quick reference

🔧 Technical Implementation

The FFI platform detection now supports HarmonyOS through multiple detection methods:

// Primary detection via platform OS
Platform.operatingSystem == 'ohos'

// Environment variable detection
OHOS_SDK_HOME, HARMONY_HOME, OHOS_NDK_HOME

// Version string analysis
Platform.version contains 'harmonyos', 'ohos', or 'harmony'

Native library loading seamlessly handles HarmonyOS alongside Android:

final _nativeLib = Platform.isAndroid || _isHarmonyOS
    ? DynamicLibrary.open('libstockfish.so')
    : DynamicLibrary.process();

Compatibility

• HarmonyOS Next: API Level 10+ (DevEco Studio 5.0+)
• Architectures: ARM64, ARM32, x86_64 simulator
• Flutter: 3.0+ with HarmonyOS support
• License: Maintains GPL v3 compatibility with Stockfish engine

Testing

The implementation reuses the proven Stockfish C++ source code and FFI bindings from Android/iOS platforms, ensuring reliability and consistency. The example app demonstrates proper integration patterns for HarmonyOS developers.

This enables chess applications and engines built with Flutter to reach the growing HarmonyOS ecosystem while maintaining code consistency across all supported platforms.

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