instrument: deliver continuous playback params live to sounding voices via a seqlock block, holding normalized stage position across time edits

src/core/instrument/engine/play_params.h

@@ -14,6 +14,8 @@
 
 namespace reasampler {
 
+namespace instrument::engine { class LiveParams; }  // live_params.h; SampleData holds a pointer
+
 using audio::AudioSample;
 using instrument::engine::VelocityCurve;
 
@@ -166,6 +168,14 @@ struct SampleData {
 
     PlayParams play;
 
+    // The live-parameter block a sounding voice tracks, or null for the bare latched engine
+    // (the default — with no block attached the core is byte-identical to the pre-live one).
+    // NON-OWNING and deliberately not per-snapshot: the shell owns ONE block that outlives
+    // every instrument snapshot, so a voice still ringing out of the drain slot follows the
+    // 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;
+
     // A framesR of a different length than frames is treated as absent — a malformed pair
     // never half-plays.
     int channelCount() const {