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.

src/shell/instrument/editor_paint_waveform.cpp

@@ -24,10 +24,14 @@ using namespace reasampler::instrument::ui;  // lanes + waveform geometry
 using audio::computeEnvelope;
 
 namespace {
-// Marker roles — semantic, drawn through the kit's palette: start = teal (secondary), loop
-// start/end = purple (tertiary). The loop-span fill is a faint purple.
+// Marker roles — semantic, drawn through the kit's palette: start AND loop start/end both
+// = teal (secondary). Loop markers moved off tertiary/purple so the loop span + bars no
+// longer share a role with the envelope overlay trace, which owns tertiary exclusively in
+// this region (measured RGB was identical: #C2AAE8 for both). Secondary-vs-primary contrast
+// here measures 1.12:1, below tertiary's 1.37:1 — accepted deliberately: markers are bars
+// and a span fill, not the 1px trace that pairing was moved off of.
 constexpr Role kRoleStartMarker = Role::AccentSecondary;
-constexpr Role kRoleLoopMarker  = Role::AccentTertiary;
+constexpr Role kRoleLoopMarker  = Role::AccentSecondary;
 }  // namespace
 
 void ReaSamplerEditor::paintWaveform(LICE_IBitmap* bmp, const Rect& band) {