Migrate Linux player from GStreamer Java/JNA to native C via JNI

[kdroidFilter]

Summary

• Replace GStreamer Java/JNA with a native C library (NativeVideoPlayer.c) using GStreamer C API directly (playbin + appsink + scaletempo + level), matching the macOS/Windows JNI architecture
• Remove JNA dependency entirely from the project — platform detection now uses System.getProperty("os.name") via CurrentPlatform utility
• Remove gst1-java-core dependency — GStreamer is now used exclusively through the native C layer
• Rewrite LinuxVideoPlayerState with coroutine-driven polling, AtomicLong native pointer, double-buffered Skia bitmaps, and output scaling — identical pattern to MacVideoPlayerState
• Add dedicated bus polling thread using gst_bus_timed_pop (no GLib main loop needed)
• Add queue element between decoder and appsink for thread decoupling
• Native build via CMake + build.sh, producing libNativeVideoPlayer.so embedded in JAR resources

Test plan

• Play local video files (MP4, WebM) on Linux
• Play HTTP/HLS streams on Linux
• Verify audio playback and volume control
• Verify seek works correctly
• Verify playback speed changes (0.5x - 2.0x)
• Verify loop mode
• Verify audio level meters (left/right)
• Verify metadata extraction (title, bitrate, dimensions, frame rate)
• Verify output scaling (resize window, check memory usage with 4K video)
• Verify fullscreen toggle
• Verify subtitle overlay
• Verify no regression on macOS build
• Verify no regression on Windows build