Make spline EG drags audible live: held and new notes follow a dragged knot

Contours ride their own seqlock block into a double-buffered engine copy, and the swap step is smoothed. Each drag move publishes contours without a model write; release still commits and reloads.

src/shell/instrument/processor_state.cpp

@@ -293,6 +293,14 @@ AutomationChannel::ReleaseProof ReaSamplerProcessor::publishLiveParams() {
     return AutomationChannel::ReleaseProof::fromPublish(before, liveParams_.generation());
 }
 
+void ReaSamplerProcessor::publishLiveContours(const PlaySeconds& play) {
+    // Folded outside the lock, as publishLiveParams' block is. A contour is normalized time, so
+    // unlike that block it needs no rate and publishes even before anything is built.
+    const instrument::engine::LiveContours contours = instrument::engine::foldContours(play);
+    std::lock_guard<std::mutex> lock(livePublishMutex_);
+    liveContours_.publish(contours);
+}
+
 SampleRefs ReaSamplerProcessor::sampleRefs() {
     std::lock_guard<std::mutex> lock(refsMutex_);
     return sampleRefs_;