fix: restore waveform symmetry about the midline, cut deck_values' link to the bank model, and unit-test the column arithmetic

The waveform column's vertical extents move to pure component_geometry so the shared
primitive stops being untested; PlaySeconds hoists into a header-only play_seconds target.

src/core/ui/component_geometry.cpp

@@ -97,4 +97,31 @@ int waveformColumnCount(const KitBox& box) {
     return w > 0 ? w : 0;
 }
 
+WaveformBand waveformBand(int bandTop, int bandHeight) {
+    WaveformBand b;
+    b.top = bandTop;
+    b.height = bandHeight;
+    b.midY = bandTop + bandHeight / 2;
+    b.halfSpan = (bandHeight / 2) - 2;  // matches drawWaveform's 2px vertical breathing room
+    return b;
+}
+
+WaveformColumnSpan waveformColumnSpan(const WaveformBand& band,
+                                      double compressedMax, double compressedMin) {
+    const int lo = band.top;
+    const int hi = band.top + band.height - 1;
+
+    WaveformColumnSpan s;
+    s.top = band.midY - static_cast<int>(compressedMax * band.halfSpan);
+    s.bottom = band.midY - static_cast<int>(compressedMin * band.halfSpan);
+    s.topF = band.midY - compressedMax * band.halfSpan;
+    s.bottomF = band.midY - compressedMin * band.halfSpan;
+
+    if (s.top < lo) s.top = lo;
+    if (s.bottom > hi) s.bottom = hi;
+    if (s.topF < lo) s.topF = lo;
+    if (s.bottomF > hi) s.bottomF = hi;
+    return s;
+}
+
 } // namespace reasampler::ui

src/core/ui/component_geometry.h

@@ -75,11 +75,38 @@ ListRowBox computeListRow(const KitBox& list, int index, int rowHeight);
 // `rowCount` rows). rowCount bounds the hit so blank space past the last row is a clean miss.
 int hitTestListRow(int px, int py, const KitBox& list, int rowHeight, int rowCount);
 
-// --- Waveform column count ---------------------------------------------------
+// --- Waveform columns --------------------------------------------------------
 
 // Pixel columns drawWaveform renders inside `box` (its fixed 2px side insets), never negative.
 // Pass directly as peaks::computeEnvelope's binCount — one bin per column is correct resolution;
 // overbinning doesn't improve render quality and wastes memory/CPU.
 int waveformColumnCount(const KitBox& box);
 
+// One channel band's shared vertical metrics: the zero line every column mirrors about, and the
+// pixel height a full-scale amplitude reaches (inset so a peak keeps the band's breathing room).
+struct WaveformBand {
+    int    top      = 0;
+    int    height   = 0;
+    int    midY     = 0;
+    double halfSpan = 0.0;
+};
+WaveformBand waveformBand(int bandTop, int bandHeight);
+
+// The vertical extents of one column: the integer edges of the filled span, and the same edges
+// at sub-pixel precision for the antialiased outline stroke that joins a column's extremes to
+// its neighbour's. Both take the band clamp, so the stroke cannot leave the band the fill is
+// confined to. Amplitudes arrive already through the display curve.
+struct WaveformColumnSpan {
+    int    top     = 0;
+    int    bottom  = 0;
+    double topF    = 0.0;
+    double bottomF = 0.0;
+};
+
+// Truncation is applied to the SCALED AMPLITUDE and then mirrored about midY — never to the
+// resulting y, which would round the two edges in opposite directions and leave a column with
+// |max| == |min| a pixel taller above the zero line than below.
+WaveformColumnSpan waveformColumnSpan(const WaveformBand& band,
+                                      double compressedMax, double compressedMin);
+
 } // namespace reasampler::ui