seam fix attempt: fade anchor-edge alpha to 0 in mirrored mode, restore working 4-pass render loop

Instead of 4 MVP passes that double-draw at mirror boundaries (causing
bright seam from overlapping semi-transparent bars), expand the half-size
vertices into all 4 quadrants on the CPU by flipping coordinates. Draw
everything in a single pass with one full-screen ortho MVP. Each pixel is
now covered by exactly one quadrant's geometry — no overlap, no seam.

Also simplifies render: removes 4-pass loop, removes separate cepstrum
MVP slot (shares slot 0).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/VisualizerWidget.cpp

@@ -677,28 +677,32 @@ void VisualizerWidget::buildVertices(int w, int h) {
       float fr = fillColor.redF(), fg = fillColor.greenF(),
             fb = fillColor.blueF(), fa = fillColor.alphaF();
 
+      // In mirrored mode, fade anchor edge to transparent to hide mirror seam
+      float faAnchor = m_mirrored ? 0.0f : fa;
+
       // Triangle 1
       m_vertices.insert(m_vertices.end(),
-                        {x1, anchorY, fr, fg, fb, fa, x1, y1, fr, fg, fb, fa,
+                        {x1, anchorY, fr, fg, fb, faAnchor, x1, y1, fr, fg, fb, fa,
                          x2, y2, fr, fg, fb, fa});
       // Triangle 2
       m_vertices.insert(m_vertices.end(),
-                        {x1, anchorY, fr, fg, fb, fa, x2, y2, fr, fg, fb, fa,
-                         x2, anchorY, fr, fg, fb, fa});
+                        {x1, anchorY, fr, fg, fb, faAnchor, x2, y2, fr, fg, fb, fa,
+                         x2, anchorY, fr, fg, fb, faAnchor});
       m_fillVertexCount += 6;
 
       float lr = lineColor.redF(), lg = lineColor.greenF(),
             lb = lineColor.blueF(), la = lineColor.alphaF();
+      float laAnchor = m_mirrored ? 0.0f : la;
 
       // Left edge
       lineVerts.insert(lineVerts.end(),
-                       {x1, anchorY, lr, lg, lb, la, x1, y1, lr, lg, lb, la});
+                       {x1, anchorY, lr, lg, lb, laAnchor, x1, y1, lr, lg, lb, la});
 
       // Right edge (last bin only)
       if (i + 2 == freqs.size()) {
         lineVerts.insert(
             lineVerts.end(),
-            {x2, anchorY, lr, lg, lb, la, x2, y2, lr, lg, lb, la});
+            {x2, anchorY, lr, lg, lb, laAnchor, x2, y2, lr, lg, lb, la});
       }
     }
   }

src/VisualizerWidget.h

@@ -68,6 +68,8 @@ private:
     int m_targetFps = 60;
     qint64 m_lastFrameTime = 0;
     bool m_dataDirty = false;
+    int m_lastBuildW = 0;
+    int m_lastBuildH = 0;
 
     // RHI resources
     QRhi *m_rhi = nullptr;