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.

src/shell/instrument/editor_input_chrome.cpp

@@ -1,6 +1,6 @@
 // editor_input_chrome.cpp — the CHROME band's input: the Browse nav, the preview trigger,
-// the preview-velocity knob grab, the curve-button summon, the channel toggle, and the
-// piano strip's root grab plus its live drag. Windows-only.
+// the preview-velocity knob grab, the channel toggle, and the piano strip's root grab plus
+// its live drag. Windows-only.
 
 #include "shell/instrument/reasampler_editor.h"
 
@@ -51,12 +51,6 @@ bool ReaSamplerEditor::mouseDownChrome(const FaceLayout& fl, int x, int y) {
         invalidate();
         return true;
     }
-    // The mini curve-preview button: summon the popup editor.
-    if (contains(cr.curveBtn, x, y)) {
-        curvePopupOpen_ = true;
-        invalidate();
-        return true;
-    }
     if (contains(cr.chanMono, x, y)) {
         channelMode_ = ChannelMode::Mono;
         processor_->setChannelMode(ChannelMode::Mono);
@@ -105,7 +99,6 @@ ReaSamplerEditor::HoverTarget ReaSamplerEditor::hoverChrome(const FaceLayout& fl
     if (selectedId_.empty()) return {};  // empty state — no interactive surfaces beyond nav
     if (contains(cr.preview, x, y)) return {HoverKind::kPreview, -1};
     if (contains(cr.velCell, x, y)) return {HoverKind::kVelKnob, -1};
-    if (contains(cr.curveBtn, x, y)) return {HoverKind::kCurveButton, -1};
     if (contains(cr.chanMono, x, y)) return {HoverKind::kChanMono, -1};
     if (contains(cr.chanStereo, x, y)) return {HoverKind::kChanStereo, -1};
     if (!cr.rootStrip.empty()) {