instrument: reassign loop-marker role off tertiary, onto secondary

Loop span/markers and the envelope overlay trace shared tertiary purple in the same
overlay rect. Loop markers now draw secondary; envelope overlay keeps tertiary.
Adds a theme-level distinctness regression guard.

tests/test_theme.cpp

@@ -147,6 +147,16 @@ static void testOverlayAccentSeparatesFromTheWaveformAccent() {
     CHECK(delta >= 60);
 }
 
+// editor_paint_waveform.cpp draws the loop span/markers and the envelope overlay trace into
+// the SAME overlay rect. Loop markers are secondary (this track's reassignment, off tertiary,
+// which the envelope overlay now owns exclusively there); a future palette edit that collapses
+// the two back to one role must fail here rather than silently recreating the finding.
+static void testLoopMarkerAndEnvelopeOverlayRolesAreDistinct() {
+    const KitColor loopMarker = roleColor(Role::AccentSecondary);
+    const KitColor envelopeOverlay = roleColor(Role::AccentTertiary);
+    CHECK(!(loopMarker == envelopeOverlay));
+}
+
 static void testWarnClearsStateFloorOnBackground() {
     // warn (destructive) must be unmistakable -> clears the state floor on the base.
     CHECK(contrastRatio(roleColor(Role::Warn), roleColor(Role::BgBase))
@@ -264,6 +274,7 @@ int main() {
     testTextOnHoverSurfaceClearsFloor();
     testSecondaryTertiaryAreDistinguishable();
     testOverlayAccentSeparatesFromTheWaveformAccent();
+    testLoopMarkerAndEnvelopeOverlayRolesAreDistinct();
     testWarnClearsStateFloorOnBackground();
     testLabelOnActiveSurfaceClearsFloor();
     testRolesAreDistinctAndElevationMonotonic();