instrument: fix AHD node-tracking/tie-break/live-latch defects and close staged-envelope-curve test gaps

src/core/instrument/engine/envelopes.h

@@ -346,14 +346,14 @@ public:
     // Trigger amp and filter envelopes. A zero/negative span finishes immediately.
     void configure(std::int64_t spanFrames, const AhdParams& params) {
         span_ = spanFrames > 0 ? spanFrames : 0;
-        fit(params);
+        fit(params, /*latchFinished=*/false);  // a fresh note starts from a clean read
         smooth_.clear();
     }
 
     // Peer of AdsrEnvelope::snapLive: a voice that has rendered nothing takes the new shape
     // outright, with no step to absorb.
     void snapLive(const AhdParams& params) {
-        fit(params);
+        fit(params, /*latchFinished=*/false);
         smooth_.clear();
     }
 
@@ -361,7 +361,10 @@ public:
     // why this smooths rather than holding a normalized position.
     void applyLive(double sourceOffset, const AhdParams& params) {
         const double before = ahdLevelAt(sourceOffset, fit_, attackCurve_, decayCurve_);
-        fit(params);
+        // LATCHED: a voice already read past its fitted total must never resurge because a
+        // later live move reopened the total. Reachable on any active() voice, including one
+        // ringing out past its own end (voice.h) where tickAmplitude() still runs.
+        fit(params, /*latchFinished=*/true);
         const double after = ahdLevelAt(sourceOffset, fit_, attackCurve_, decayCurve_);
         if (after != before) smooth_.absorb(before - after);
     }
@@ -381,11 +384,15 @@ public:
     const AhdSpan& stages() const { return fit_; }
 
 private:
-    void fit(const AhdParams& p) {
+    // `latchFinished`: once true, a re-fit can only ever KEEP finished_ true, never clear it —
+    // see applyLive above for why. configure()/snapLive() pass false: those are a fresh read
+    // (new note or a not-yet-rendered voice), which must compute finished_ from scratch.
+    void fit(const AhdParams& p, bool latchFinished) {
         fit_ = fitAhd(span_, p);
         attackCurve_ = p.attackCurve;
         decayCurve_ = p.decayCurve;
-        finished_ = (fit_.total <= 0);
+        const bool empty = (fit_.total <= 0);
+        finished_ = latchFinished ? (finished_ || empty) : empty;
     }
 
     std::int64_t span_ = 0;