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_paint_chrome.cpp

@@ -129,12 +129,22 @@ void ReaSamplerEditor::paintChrome(LICE_IBitmap* bmp, const FaceLayout& fl, bool
     if (empty) return;
 
     // Preview-trigger button (fires the loaded capture at root through the live voice engine).
+    // A drawn play triangle rather than a label or an embedded image: it inherits the button's
+    // own foreground role, so it stays legible in every interaction state at no build cost.
     {
         const KitButtonBox box{toKitBox(cr.preview)};
-        const InteractionState st = (previewingNote_ >= 0) ? InteractionState::Active
+        const bool active = (previewingNote_ >= 0);
+        const InteractionState st = active ? InteractionState::Active
             : (isHovered(HoverKind::kPreview, -1) ? InteractionState::Hover
                                                   : InteractionState::Rest);
-        drawButton(bmp, box, "Preview", st, /*warn=*/false);
+        drawButton(bmp, box, nullptr, st, /*warn=*/false);
+        const PreviewGlyph g = previewGlyph(cr.preview);
+        if (!g.empty()) {
+            const LICE_pixel ink =
+                toLice(roleColor(active ? Role::BgBase : Role::TextPrimary));
+            LICE_FillTriangle(bmp, g.leftX, g.topY, g.leftX, g.bottomY, g.apexX, g.apexY,
+                              ink, 1.0f, 0);
+        }
     }
 
     // Preview velocity: a radial knob cell (the deck cell grammar), bound to the same
@@ -156,9 +166,6 @@ void ReaSamplerEditor::paintChrome(LICE_IBitmap* bmp, const FaceLayout& fl, bool
         }
     }
 
-    // The mini curve-preview button: opens the popup editor.
-    paintCurveButton(bmp, cr.curveBtn);
-
     // Mono | Stereo output-mode toggle.
     {
         const bool isStereo = (channelMode_ == ChannelMode::Stereo);