Ω-W1-T5: one overlay↔waveform mapping, and loop marks that tell the truth about their mode

tests/test_component_geometry.cpp

@@ -192,6 +192,16 @@ static void testWaveformColumnCount() {
     CHECK(waveformColumnCount(KitBox{0, 0, 0, 40}) == 0);
 }
 
+// The instrument's waveform overlay rides this exact column band, and derives its own left
+// inset by halving what this leaves (waveform_view's waveformOverlayArea) rather than keeping a
+// second copy of the inset. That halving is only correct while the inset is symmetric, so pin
+// it: every drawable width loses exactly 4, two per side.
+static void testTheColumnBandIsInsetSymmetrically() {
+    for (int w = 5; w <= 400; ++w) {
+        CHECK(w - waveformColumnCount(KitBox{0, 0, w, 40}) == 4);
+    }
+}
+
 // --- waveform column span ----------------------------------------------------
 
 // The regression this exists to catch: rounding applied to the resulting y instead of to the
@@ -293,6 +303,7 @@ int main() {
     testListRowHitTestBoundedByCount();
     testListRowLayoutHitAgreement();
     testWaveformColumnCount();
+    testTheColumnBandIsInsetSymmetrically();
     testSymmetricColumnDrawsEqualHeightAboveAndBelowTheZeroLine();
     testSilentColumnCollapsesOntoTheZeroLine();
     testTallerAmplitudeNeverDrawsAShorterColumn();