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/reasampler_processor.h

@@ -23,6 +23,7 @@
 #include "core/instrument/map/sample_map.h"     // InstrumentParams (the one parameter set)
 #include "core/instrument/map/component_state_io.h"  // ComponentState codec
 #include "core/instrument/engine/limiter.h"      // the master bus's post-gain limiter
+#include "core/instrument/engine/live_contours.h"  // LiveContourBlock (the drawn-contour block)
 #include "core/instrument/engine/live_params.h"  // LiveParams (the live-parameter block)
 #include "core/instrument/engine/meter_accumulate.h"  // the meter's block-rate folds + consume
 #include "core/instrument/param/param_id.h"      // the frozen ParamId space + DeckParam binding
@@ -31,6 +32,7 @@ namespace reasampler::vst {
 
 using instrument::map::ComponentState;
 using instrument::map::InstrumentParams;
+using instrument::map::PlaySeconds;
 using instrument::map::SampleRefEntry;
 using instrument::map::SampleRefs;
 using instrument::map::kPreviewVelocityDefault;
@@ -206,6 +208,11 @@ public:
     // bakes and publishes). UI thread; serialized against reloadInstrument's own publish. Returns drainAutomationToModel's release proof (automation_channel.h); other callers ignore it.
     AutomationChannel::ReleaseProof publishLiveParams();
 
+    // Publishes `play`'s three drawn contours to every sounding voice WITHOUT writing the
+    // model — the contour drag's courier (this directory's CLAUDE.md, the authority table).
+    // UI thread; serialized against reloadInstrument's own contour publish.
+    void publishLiveContours(const PlaySeconds& play);
+
     // Per-instance channel mode (mono | stereo), guarded by channelModeMutex_, never read
     // on the audio thread. Decode policy only (downmix vs L/R split) — the output bus is
     // fixed stereo, so a mode change never renegotiates host I/O.
@@ -387,6 +394,10 @@ private:
     // Both live_ and draining_ observe this same block — a block owned by a snapshot would
     // leave the drain's still-sounding voices deaf to the knob under them.
     instrument::engine::LiveParams liveParams_;
+    // The drawn-contour block, SampleData::liveContours' one target, under liveParams_' ordering
+    // constraint and for its reason. No automation merge: no contour is a host parameter, so both
+    // engines read the model/editor publish directly.
+    instrument::engine::LiveContourBlock liveContours_;
     // The block the ENGINE reads, and the ONE thing SampleData::live points at. Written only by
     // the audio thread, which merges liveParams_ with the host's automation points once per block
     // and republishes ONLY when the RESULT moved — so neither an unchanged model nor a lane
@@ -404,10 +415,10 @@ private:
     // The host lane's state and its release protocol; automation_channel.h owns the mechanism and
     // this directory's CLAUDE.md the authority model it enforces.
     AutomationChannel automation_;
-    // Serializes liveParams_.publish's two writer sites (reloadInstrument, publishLiveParams)
-    // only — separate from reloadMutex_ so a knob drag's publish never blocks behind a
-    // reload's WAV decode. The audio thread never takes this; process() only reads via
-    // LiveParams::read's lock-free seqlock retry.
+    // Serializes the writer sites of liveParams_ (reloadInstrument, publishLiveParams) and of
+    // liveContours_ (reloadInstrument, publishLiveContours) only — separate from reloadMutex_ so
+    // a drag's publish never blocks behind a reload's WAV decode. The audio thread never takes
+    // this; it only reads via the seqlock's lock-free retry.
     std::mutex livePublishMutex_;
     // The rate the loaded capture was decoded/built at, so a live republish resolves the
     // stored wall-clock seconds to exactly the frames the built SampleData carries. 0 = nothing