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/play_params.h

@@ -15,7 +15,8 @@
 
 namespace reasampler {
 
-namespace instrument::engine { class LiveParams; }  // live_params.h; SampleData holds a pointer
+// SampleData holds a pointer to each (live_params.h, live_contours.h).
+namespace instrument::engine { class LiveParams; class LiveContourBlock; }
 
 using audio::AudioSample;
 using instrument::engine::VelocityCurve;
@@ -181,8 +182,8 @@ struct PlayParams {
     // The three drawn contours: the alternative to adsr/trigAhd, to pitchEnv.shape, and to
     // filter.env/trigEnv respectively. They sit HERE rather than inside the three envelope
     // structs because those are copied whole into the live block, which must stay trivially
-    // copyable (live_params.h) — and a contour is not a live control anyway: like the velocity
-    // curves it travels by reload.
+    // copyable (live_params.h). A contour reaches a sounding voice through its own block
+    // instead (live_contours.h); only its Staged|Spline mode travels by reload.
     SplineEnv      ampSpline;
     SplineEnv      pitchSpline;
     SplineEnv      filterSpline;
@@ -295,6 +296,9 @@ struct SampleData {
     // same knob as a live one. That is the desired behaviour — it is the note the user is
     // hearing. Do not "fix" it by moving ownership into the snapshot.
     const instrument::engine::LiveParams* live = nullptr;
+    // The drawn-contour peer of `live`, under the same ownership and null contract: null plays
+    // the contours in `play` for the note's whole life.
+    const instrument::engine::LiveContourBlock* liveContours = nullptr;
 
     // A framesR of a different length than frames is treated as absent — a malformed pair
     // never half-plays.