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

src/core/instrument/engine/voice_engine.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "core/audio/peaks.h"
+#include "core/instrument/engine/live_params.h"
 #include "core/instrument/engine/play_params.h"
 #include "core/instrument/engine/voice.h"
 
@@ -106,6 +107,23 @@ private:
     // one per the documented policy. Always returns a valid index (maxVoices >= 1).
     std::size_t allocateVoice();
 
+    // --- Live-parameter observation (live_params.h) ---
+    // The ONE place the seqlock is read: at block start and at each note-on, on the audio
+    // thread, never per frame. A torn or never-published read leaves the last good snapshot
+    // in place rather than spinning. Returns whether a NEW generation landed.
+    bool refreshLive();
+    // Block-boundary refresh: pushes a newly-observed generation into every sounding voice,
+    // which glides toward it. No-op when nothing changed (and when no block is attached).
+    void applyLiveToActive();
+    // The one restart path: start the voice, hand it the live values outright (it has nothing
+    // to glide from), and stamp its age. Shared by the poly steal and both mono restarts so
+    // no restart site can miss the live handoff.
+    void startVoice(Voice& voice, int note, int velocity);
+
+    instrument::engine::LiveValues live_{};
+    std::uint32_t liveGeneration_ = 0;  // last generation observed; 0 = none yet
+    bool haveLive_ = 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;