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.

tests/test_sample_chrome.cpp

@@ -8,6 +8,7 @@
 // toolbar overlapping any other; degenerate bands yielding no inverted rects; and the preview
 // button's play-triangle glyph, which sits inside the button without changing its rect.
 
+#include "../src/core/instrument/ui/knob_deck.h"
 #include "../src/core/instrument/ui/sample_bands.h"
 #include "../src/core/instrument/ui/sample_chrome.h"
 
@@ -21,7 +22,9 @@ static int g_fail = 0;
 #define CHECK(cond) do { if(!(cond)) { \
     std::printf("FAIL line %d: %s\n", __LINE__, #cond); ++g_fail; } } while(0)
 
-static constexpr int kKnob = 40;  // stands in for knob_deck's kDeckKnobSize
+// The real constant, not a copy: the shell passes kDeckKnobSize into chromeRects, and a
+// hand-copied stand-in here had already drifted from it once.
+static constexpr int kKnob = kDeckKnobSize;
 
 static Rect chromeBand(int w = kEditorMinWidth, int h = kEditorMinHeight) {
     return computeSampleBands(w, h, 120).chrome;