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