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/core/instrument/engine/voice_engine.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "core/audio/peaks.h"
+#include "core/instrument/engine/live_contours.h"
 #include "core/instrument/engine/live_params.h"
 #include "core/instrument/engine/play_params.h"
 #include "core/instrument/engine/voice.h"
@@ -57,6 +58,11 @@ public:
                 MonoTrigger monoTrigger = MonoTrigger::Retrigger,
                 bool takeoverDeclick = false);
 
+    // Voices may hold raw pointers into this engine's own contour buffers, so a copy would
+    // leave them reading the original's.
+    VoiceEngine(const VoiceEngine&) = delete;
+    VoiceEngine& operator=(const VoiceEngine&) = delete;
+
     // MIDI note-on. Allocates a free voice, or steals one per the policy above. Returns the
     // index of the voice used, or kNoVoice when nothing is playable (no decoded PCM, an
     // out-of-range note, or a Preserve note-on past the cap) — a defined no-play, not an error.
@@ -124,6 +130,22 @@ private:
     std::uint32_t liveGeneration_ = 0;  // last generation observed; 0 = none yet
     bool haveLive_ = false;
 
+    // --- Drawn-contour observation (live_contours.h) ---
+    // Reads a new contour generation, when one landed, into the buffer no voice is bound to and
+    // rebinds EVERY active voice to it before returning — which is what keeps the other buffer
+    // free for the next read. That invariant is why this, and never a bare read, is what both
+    // the block boundary and startVoice call.
+    void applyContoursToActive();
+
+    // Double-buffered because a voice's SplineCursor points INTO the buffer: reading over it
+    // would change a contour under a sounding voice without the rebind that absorbs the step,
+    // and a torn read (which the seqlock only reports after the fact) must land somewhere no
+    // voice reads. ~9 KB each, in the engine, so nothing on the audio thread allocates.
+    std::array<instrument::engine::LiveContours, 2> contours_{};
+    std::size_t contourFront_ = 0;
+    std::uint32_t contourGeneration_ = 0;  // last generation observed; 0 = none yet
+    bool haveContours_ = false;
+
     // Count of active Preserve-engine voices (for the Preserve cap). Rescanned per note-on
     // (cheap: bounded by maxVoices) rather than maintained as a running tally.
     std::size_t activePreserveVoices() const;