otio: write importable OTIO/FCPXML (DaVinci Resolve rejects both)

- available_range declared a zero duration for every clip (the media
  was 'unusable', so Resolve invalidated all of them): it now covers
  the whole media - video clips in the probed frame rate, audio clips
  in the probed sample rate (no more hardcoded 48000/0).
- Gap ranges mixed time bases (a 24 fps block start, a 1.0 trailing
  duration): gaps now stay in the sequence's time base, the trailing
  pad in whole frames.
- Empty sequences (no clips) are skipped, so a lone edited sequence
  exports as a bare Timeline root instead of a SerializableCollection
  Resolve cannot read.
- FCPXML: an asset used on both video and audio tracks now declares
  hasVideo AND hasAudio (the first registration's track kind used to
  hide the other stream); asset durations recover with available_range.

crates/oak-task/src/nodeops.rs

@@ -472,6 +472,19 @@ pub fn footage_video_params(
 	f.video_params(index).map(common_from_node_video)
 }
 
+/// The footage's first audio stream's sample rate (`oaknode_footage_get_
+/// audio_params`), `None` when unprobed or the file has no audio stream.
+pub fn footage_audio_sample_rate(project: &ProjectRef, footage: NodeId) -> Option<i32> {
+	let guard = lock_project(project);
+	let f = guard
+		.graph
+		.get(footage)?
+		.behavior
+		.as_any()?
+		.downcast_ref::<FootageBehavior>()?;
+	f.audio_params(0).map(|a| a.sample_rate)
+}
+
 /// Overwrite (or create) the footage's `index`th video stream parameters
 /// (`oaknode_footage_set_video_params`).
 pub fn footage_set_video_params(