app: route all FFmpeg access through the ffmpeg_bridge C API

Replace every direct FFmpeg call in the editor and the render worker
with the pure C ffmpeg_bridge API, wrapped in thin C++ adapters that
preserve the original interfaces:

- avframeptr.h: olive::AVFrame adapter around FBFrame handles
- ffmpegutils: format conversion helpers on FB_* constants; the int
  overload is renamed GetCompatibleBridgePixelFormat to avoid a silent
  overload-resolution trap with the PixelFormat enum
- ffmpegdecoder/ffmpegencoder: rewritten as handle-based adapters over
  FBDecoder/FBProbe/FBEncoder/FBScaler/FBResampler
- audioprocessor: FBAudioGraph push/pull adapter
- pluginrenderer/OliveClip: sws/pixdesc usage converted to FBScaler and
  fb_pix_fmt_* queries
- AudioParams/channel layouts are plain uint64_t masks everywhere

Build integration: the root project no longer links FFMPEG directly;
only ffmpeg_bridge does. Binaries resolve the bridge library at runtime
via @loader_path inside the macOS app bundle (copied there post-build)
and via $ORIGIN/../ffmpeg_bridge/bin on Linux; on Windows the DLL is
installed next to the executables, so packages on all three platforms
ship the bridge library.

ffmpeg_bridge gains the extra API the adapters need:
fb_frame_make_writable, fb_decoder_get_format_duration,
fb_resampler_convert_frame, FB_PIX_FMT_YUV440P, SRT validation in
fb_probe_read_subtitle_stream, packed/planar fixes in
fb_encoder_write_audio, and a component-size fix in
fb_pix_fmt_component_size.

ffmpeg_bridge/src/swr.cpp

@@ -81,3 +81,14 @@ int fb_resampler_convert(FBResampler *resampler, uint8_t **out, int out_count,
 	}
 	return swr_convert(resampler->ctx, out, out_count, in, in_count);
 }
+
+int fb_resampler_convert_frame(FBResampler *resampler, uint8_t **out,
+							   int out_count, const FBFrame *in_frame)
+{
+	if (!resampler || !in_frame || !in_frame->frame) {
+		return AVERROR(EINVAL);
+	}
+	return swr_convert(resampler->ctx, out, out_count,
+					   (const uint8_t **)in_frame->frame->extended_data,
+					   in_frame->frame->nb_samples);
+}