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.

app/config/config.cpp

@@ -204,7 +204,8 @@ void Config::SetDefaults()
 	SetEntryInternal(QStringLiteral("AudioOutputSampleRate"), NodeValue::kInt,
 					 48000);
 	SetEntryInternal(QStringLiteral("AudioOutputChannelLayout"),
-					 NodeValue::kInt, AV_CH_LAYOUT_STEREO);
+					 NodeValue::kInt,
+					 QVariant::fromValue(static_cast<int64_t>(kChannelLayoutStereo)));
 	SetEntryInternal(
 		QStringLiteral("AudioOutputSampleFormat"), NodeValue::kText,
 		QString::fromStdString(SampleFormat(SampleFormat::S16).to_string()));
@@ -216,7 +217,8 @@ void Config::SetDefaults()
 	SetEntryInternal(QStringLiteral("AudioRecordingSampleRate"),
 					 NodeValue::kInt, 48000);
 	SetEntryInternal(QStringLiteral("AudioRecordingChannelLayout"),
-					 NodeValue::kInt, AV_CH_LAYOUT_STEREO);
+					 NodeValue::kInt,
+					 QVariant::fromValue(static_cast<int64_t>(kChannelLayoutStereo)));
 	SetEntryInternal(
 		QStringLiteral("AudioRecordingSampleFormat"), NodeValue::kText,
 		QString::fromStdString(SampleFormat(SampleFormat::S16).to_string()));
@@ -251,7 +253,7 @@ void Config::SetDefaults()
 					 NodeValue::kInt, 48000);
 	SetEntryInternal(
 		QStringLiteral("DefaultSequenceAudioLayout"), NodeValue::kInt,
-		QVariant::fromValue(static_cast<int64_t>(AV_CH_LAYOUT_STEREO)));
+		QVariant::fromValue(static_cast<int64_t>(kChannelLayoutStereo)));
 
 	// Online/offline settings
 	SetEntryInternal(QStringLiteral("OnlinePixelFormat"), NodeValue::kInt,