feat(ffmpeg): migrate from CLI bridge to libav*-based Kotlin API with CLI fallback

[GeneralK1ng]

Summary

This PR migrates mediamp-ffmpeg from a pure CLI bridge (FFmpegKit.execute(args) calling main()) to a thin Kotlin Multiplatform wrapper around libavformat / libavcodec, while preserving the CLI bridge as a fallback for unmapped operations.

Key Changes

New Kotlin API

• MediaTranscoder + MediaOperation for high-level operations:
  • MediaOperation.Remux — stream copy with automatic time_base rescaling, per-stream BSF, and muxer options.
  • MediaOperation.Probe — read stream info.
• Low-level wrappers: InputContainer, OutputContainer, Stream, AVPacket, BitstreamFilter, MuxerOptions.
• Unified native types via internal expect/actual typealiases, enabling shared logic in commonMain.

CLI Bridge Preservation

FFmpegKit.execute(args) remains available as a fallback:

• Recognized operations (Remux, Probe) are routed through the new Kotlin API.
• Unrecognized / complex operations fall back to the extracted ffmpeg binary (Android/Desktop) or throw UnsupportedOperationException (iOS).

Android Native Library Consolidation

• Switched androidMain dependency to libs.ffmpeg (pure Java API) to prevent JavaCPP prebuilt libav*.so from entering the AAR.
• Added ExtractJavaCppJniTask to extract JavaCPP JNI bridge libjni*.so from classifier jars.
• AAR now contains exactly one set of self-built libav*.so + libjni*.so, resolving mergeNativeLibs conflicts.

Breaking Changes

• Removed legacy CLI wrapper C files (ffmpegkit_jni.c, MediampFFmpegKit.h) and fftools linkage.
• Flattened package directory structure.

[StageGuard]

Found two blocking regressions in the optimized FFmpegKit path.