fix(vst): master-gain per-sample ramp (no zipper/click); voice-count commits once on release; minor review items

tests/test_master_gain.cpp

@@ -74,6 +74,21 @@ static void testNonFiniteLinearClamps() {
     CHECK(near(masterGainNormFromLinear(1e9), 1.0));  // above the cap clamps to 1
 }
 
+static void testBelowFloorCollapsesToBottom() {
+    // Any linear gain at or below the kMasterGainMinDb floor collapses to norm 0 (the -inf
+    // bottom detent). The floor IS the bottom of the finite sweep, so -61 dB, -80 dB, and
+    // a vanishingly small positive linear all map to the same place as true zero.
+    const double floorLinear = std::pow(10.0, kMasterGainMinDb / 20.0);  // 10^(-60/20) = 0.001
+    CHECK(masterGainNormFromLinear(floorLinear) == 0.0);   // exactly at the floor -> bottom
+    CHECK(masterGainNormFromLinear(floorLinear * 0.5) == 0.0);  // below the floor -> bottom
+    CHECK(masterGainNormFromLinear(1e-9) == 0.0);               // tiny positive -> bottom
+    // The dB path agrees: anything at or below kMasterGainMinDb maps to norm 0.
+    CHECK(masterGainNormFromDb(kMasterGainMinDb) == 0.0);
+    CHECK(masterGainNormFromDb(kMasterGainMinDb - 20.0) == 0.0);  // -80 dB -> bottom
+    // Just ABOVE the floor (by epsilon) returns a non-zero norm.
+    CHECK(masterGainNormFromDb(kMasterGainMinDb + 1e-9) > 0.0);
+}
+
 static void testLabels() {
     char buf[24];
     formatMasterGainLabel(0.0, buf, sizeof(buf));
@@ -93,6 +108,7 @@ int main() {
     testNormLinearRoundTripAcrossTravel();
     testMonotonic();
     testNonFiniteLinearClamps();
+    testBelowFloorCollapsesToBottom();
     testLabels();
     if (g_fail) {
         std::printf("%d FAILURE(S)\n", g_fail);