instrument: one VELOCITY deck for all three velocity curves, bipolar and off by default for pitch and filter

Payload v12 appends the new velocity->pitch curve and folds the retired filter velAmount into its now-bipolar curve, so pre-v12 projects reopen sounding identical. Preview button takes a drawn play triangle.

tests/test_live_delivery.cpp

@@ -786,14 +786,13 @@ static void testPitchRatioAndVelocityGainStayLatched() {
 
 static void testVelocityGainSurvivesAHostilePublishThatReallyLands() {
     // Filter AND pitch envelope enabled, so every field the block carries actually reaches the
-    // voice. velAmount is 0, so velocity enters the render exactly once — as the amp gain
-    // latched at note-on — which makes two runs at different velocities exactly proportional
-    // unless the publish moved that gain (a re-derived gain would have to preserve the ratio
-    // 100:64 to slip through).
+    // voice. The filter and pitch velocity curves are left at their off defaults, so velocity
+    // enters the render exactly once — as the amp gain latched at note-on — which makes two
+    // runs at different velocities exactly proportional unless the publish moved that gain (a
+    // re-derived gain would have to preserve the ratio 100:64 to slip through).
     SampleData rig = periodicSine(200000, 64.0);
     rig.velocityCurve = VelocityCurve::linear();
     filterSweep(rig);
-    rig.play.filter.velAmount = 0.0;
     rig.play.pitchEnv.enabled = true;
     rig.play.pitchEnv.shape.decayFrames = 24000;
     rig.play.pitchEnv.peakSemitones = 3.0;