instrument: latch the note done at the read-head run-off, fit the migrated fades, and lift the curve dial and overlay selection into pure modules

src/core/instrument/engine/voice.h

@@ -348,6 +348,12 @@ private:
         // play span ends within the ramp). With no declick (the common case) this is
         // byte-identical to the plain idle-out.
         if (triggerRanOff || readPos_ >= static_cast<double>(frameCount)) {
+            // The NOTE is over the moment the read head leaves its span, whether or not a ramp
+            // still rings: no later frame can carry envelope output. Latching here is what keeps
+            // a ringing-out voice out of soundingNote() — the Preserve cap would otherwise
+            // refuse a new onset, and mono legato would retune a voice already past its end
+            // (silencing the new note) for the whole ~4 ms ramp.
+            amplitudeDone_ = true;
             if (declickPending_) seedDeclick();
             if (!declickActive_) seedTerminalDeclick();
             if (declickActive_) {
@@ -376,8 +382,11 @@ private:
         // Peer of the read-head exhaustion path above: a Trigger AHD whose stages end BEFORE
         // the play span (a zero decay, which the shape deliberately keeps expressible) cuts the
         // same synthetic Preserve tail at whatever level it was at. Seeded from lastOut, which
-        // still holds the PREVIOUS frame — this one is already silent. Gate is left out on
-        // purpose: its amplitude reaches zero through a release, so there is no cut to ring out.
+        // still holds the PREVIOUS frame — this one is already silent. Gate is left out of THIS
+        // site only: its amplitude reaches zero through a release, so nothing here is cut
+        // mid-level. The exhaustion path above deliberately does NOT exclude Gate — a held Gate
+        // note whose source runs out with no loop is cut at its sustain level, and under
+        // Preserve that cut lands on the same recycled synthetic tail.
         if (amplitudeDone_ && amp == 0.0 && !declickActive_ &&
             playMode_ == PlayMode::Trigger) {
             seedTerminalDeclick();