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/core/instrument/ui/sample_chrome.h

@@ -21,7 +21,6 @@ struct ChromeRects {
     Rect velCell;     // preview-velocity knob cell (knob + label band)
     Rect velKnob;
     Rect velLabel;
-    Rect curveBtn;    // opens the velocity-curve popup
     Rect chanMono;
     Rect chanStereo;
     Rect navBrowse;
@@ -32,4 +31,20 @@ struct ChromeRects {
 // `knobSize` is the deck knob square, passed in so this module does not depend on knob_deck.
 ChromeRects chromeRects(const Rect& chrome, int knobSize);
 
+// A right-pointing play triangle centered in the preview button: the button's whole label.
+// Three vertices, handed straight to one filled-triangle draw. Drawn rather than embedded as a
+// bitmap so it inherits the palette role of whatever interaction state the button is in.
+struct PreviewGlyph {
+    int leftX = 0;   // the vertical edge
+    int topY = 0;
+    int bottomY = 0;
+    int apexX = 0;   // the point, on the button's vertical centre line
+    int apexY = 0;
+    bool empty() const { return apexX <= leftX || bottomY <= topY; }
+};
+
+// Sized off the button's shorter dimension and clamped, so the glyph stays legible in a squat
+// button and never outgrows a generous one. An unusably small button yields an empty glyph.
+PreviewGlyph previewGlyph(const Rect& button);
+
 }  // namespace reasampler::instrument::ui