fix(sampler_core): gate takeover declick by (1-amp) to remove zero-attack +6 dB blip and long-attack notch; clamp seed to +/-1; add zero-attack test

src/vst/sampler_core.cpp

@@ -272,8 +272,10 @@ void Voice::start(int note, int velocity, const SampleData& sample, int rootNote
     // the same pre-cut output level — zeroing would drop the pending ramp and bring the
     // click back on that edge. The next rendered frame overwrites lastOut anyway.
     if (declickTakeover && active_) {
-        declickL_ = lastOutL_;
-        declickR_ = lastOutR_;
+        // Clamp the seed to ±1.0: closes the theoretical same-frame-repeat accumulation edge
+        // (successive starts before any frame is rendered cannot grow the seed above full scale).
+        declickL_ = (lastOutL_ > 1.0) ? 1.0 : (lastOutL_ < -1.0) ? -1.0 : lastOutL_;
+        declickR_ = (lastOutR_ > 1.0) ? 1.0 : (lastOutR_ < -1.0) ? -1.0 : lastOutR_;
         declickActive_ = (declickL_ > kDeclickFloor || declickL_ < -kDeclickFloor ||
                           declickR_ > kDeclickFloor || declickR_ < -kDeclickFloor);
     } else {
@@ -530,9 +532,16 @@ AudioSample Voice::advanceFrame(bool stereo, AudioSample& outR) {
     // takeover/steal start() so the restart's hard cut has no step. Engine-agnostic — applied
     // after either pitch-engine branch, on the shared epilogue. Inactive (the common case)
     // costs one branch.
+    //
+    // Gate by (1 - amp): the compensation fills only the HOLE the new attack leaves. When amp
+    // is near 0 (slow attack, typical 3 ms) the gate is ~1 — full compensation, no change in
+    // feel. When amp is 1 (zero-attack, instant sustain) the gate is 0 — no compensation added,
+    // so there is no +6 dB blip. For long attacks the gate tapers the compensation proportionally,
+    // removing the notch that arose when both the old tail and the new level were present in full.
     if (declickActive_) {
-        outL += declickL_;
-        if (stereo) outRlocal += declickR_;
+        const double gate = 1.0 - amp;
+        outL += declickL_ * gate;
+        if (stereo) outRlocal += declickR_ * gate;
         declickL_ *= kDeclickDecay;
         declickR_ *= kDeclickDecay;
         if (declickL_ < kDeclickFloor && declickL_ > -kDeclickFloor &&