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/instrument/ui/knob_deck.cpp

@@ -190,24 +190,19 @@ DeckHit hitTestDeck(const DeckLayout& layout, int x, int y) {
     return {};
 }
 
-namespace {
-// Squared distance from a rect's centre, in the rect's own pixel units.
-double distSqFromCentre(const Rect& r, int x, int y) {
-    const double dx = x - (r.x + r.width / 2.0);
-    const double dy = y - (r.y + r.height / 2.0);
-    return dx * dx + dy * dy;
+bool inKnobFace(const Rect& knob, int x, int y) {
+    const double dx = x - (knob.x + knob.width / 2.0);
+    const double dy = y - (knob.y + knob.height / 2.0);
+    const double r = knob.width / 2.0;
+    return dx * dx + dy * dy < r * r;
 }
-}  // namespace
 
 DeckFaceHit hitTestKnobFace(const DeckLayout& layout, int x, int y) {
     for (const DeckGroupLayout& g : layout.groups) {
         if (!contains(g.box, x, y)) continue;
         for (const DeckCellLayout& c : g.cells) {
-            const double rOuter = c.knob.width / 2.0;
-            if (distSqFromCentre(c.knob, x, y) >= rOuter * rOuter) continue;
-            const double rInner = c.inner.width / 2.0;
-            const bool inner = distSqFromCentre(c.inner, x, y) < rInner * rInner;
-            return {c.id, inner};
+            if (!inKnobFace(c.knob, x, y)) continue;
+            return {c.id, inKnobFace(c.inner, x, y)};
         }
         return {};  // inside the group but off every dial
     }