feat(ffi)!: byte-stream producers form a simulcast ladder

cpp/moq/MIGRATE.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### `moq::VideoInit::lower` takes either producer kind ([#7699](https://github.com/moq-dev/moq/pull/7699))
+
+`lower` is `std::optional<moq::MediaProducer>` instead of a `std::shared_ptr<moq::MediaTrackProducer>`. Assign `moq::MediaProducer(moq::MediaProducer::kTrack{producer})`, or `kStream` for a `moq::MediaTrackStreamProducer`.
+
 ### Media setup refusals are typed
 
 Audio and video subscribe setup now reports `moq::Error::kUnsupportedConfig` when the decoder refuses a codec, layout, or description. Handle this variant to try another rendition instead of inspecting an audio/video error message or `Unsupported`. Decoder faults, such as a backend that fails to open, and later peer and decode failures keep their existing errors. Regenerate bindings and rebuild the matching native library together.

cpp/obs/src/moq-output.cpp

@@ -850,7 +850,7 @@ void MoQOutput::VideoInit(obs_encoder_t *encoder)
 		if (below == video_tracks.end())
 			return;
 		if (below->second.producer) {
-			config.lower = below->second.producer;
+			config.lower = moq::MediaProducer(moq::MediaProducer::kTrack{below->second.producer});
 			lower_bps = (uint64_t)lower_kbps * 1000ULL;
 		}
 	}

dart/moq/MIGRATE.md

@@ -4,6 +4,10 @@ How to update your code for each breaking change, newest first.
 
 ## Unreleased
 
+### `VideoInit.lower` takes either producer kind ([#7699](https://github.com/moq-dev/moq/pull/7699))
+
+`VideoInit.lower` is a `Producer` (`MoqMediaProducer`) instead of a `TrackProducer`. Pass `TrackMoqMediaProducer(producer: ...)`, or `StreamMoqMediaProducer(producer: ...)` for a byte-stream rendition.
+
 ### Media setup refusals are typed
 
 Audio and video subscribe setup now reports `UnsupportedConfigMoqException` when the decoder refuses a codec, layout, or description. Handle this variant to try another rendition instead of inspecting an audio/video error message or `Unsupported`. Decoder faults, such as a backend that fails to open, and later peer and decode failures keep their existing errors. Regenerate bindings and rebuild the matching native library together.

dart/moq/lib/media.dart

@@ -9,6 +9,9 @@ typedef TrackProducer = MoqMediaTrackProducer;
 /// The write side of a media track fed a raw byte stream, with frame boundaries inferred.
 typedef TrackStreamProducer = MoqMediaTrackStreamProducer;
 
+/// A single-track producer of either kind, such as a video rendition's `lower`.
+typedef Producer = MoqMediaProducer;
+
 /// The write side of a container, which publishes each track it describes.
 typedef ContainerProducer = MoqMediaContainerProducer;