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

tests/test_sampler_core.cpp

@@ -2063,6 +2063,35 @@ static void testSameBlockDoubleTakeoverKeepsDeclickSeed() {
     CHECK(approx(post.back(), 1.0, 1e-3));
 }
 
+// Declick with ZERO-ATTACK takeover: the new voice reaches full level on frame 0 (amp == 1),
+// so the gated compensation adds nothing (gate = 1 - 1 = 0). Output on the boundary frame is
+// exactly the new voice's level, never exceeding full scale. Without the (1-amp) gate a zero-
+// attack takeover from a sustained voice produced newOnset + oldLevel -> up to 2x (+6 dB).
+static void testZeroAttackTakeoverNeverExceedsFullScale() {
+    SampleData s = dcSample(200000, 60);
+    s.play.adsr.attackFrames = 0;     // zero-attack: amp == 1 on the very first frame
+    s.play.adsr.sustainLevel = 1.0;
+    s.play.adsr.releaseFrames = 0;
+    Keymap km = Keymap::singleSampleChromatic(std::move(s));
+    VoiceEngine eng(1, km, 0, 0, VoiceMode::Mono, MonoTrigger::Retrigger,
+                    /*takeoverDeclick=*/true);
+
+    eng.noteOn(60, 127);
+    std::vector<AudioSample> pre;
+    eng.render(pre, 200);             // sustained at 1.0
+    CHECK(approx(pre.back(), 1.0, 1e-6));
+
+    eng.noteOn(64, 127);              // zero-attack takeover: amp hits 1 on frame 0
+    std::vector<AudioSample> post;
+    eng.render(post, 400);
+    // Every output frame must stay within [-1, 1]: no +6 dB blip.
+    for (AudioSample v : post) {
+        CHECK(v <= 1.0f + 1e-4f && v >= -1.0f - 1e-4f);
+    }
+    // The zero-attack note settles at sustain 1.0 immediately.
+    CHECK(approx(post[0], 1.0, 1e-4));
+}
+
 // GA-VoiceSteal repro (DAW bug): voiceCount 3, a triad note-on'd at the SAME sample time
 // (three note-ons in one block, no render between), then a 4th note. The steal must take
 // EXACTLY ONE voice (the oldest, none releasing) and leave the other two RINGING — the DAW
@@ -2265,6 +2294,7 @@ int main() {
     testMonoDeclickOnlyOnTakeover();
     testPolyStealDeclicksRestart();
     testSameBlockDoubleTakeoverKeepsDeclickSeed();
+    testZeroAttackTakeoverNeverExceedsFullScale();
     testPreviewCardIsolatedFromPool();
     testPreviewCardReplaceStaleOffAndOutOfZone();