instrument: one staged-envelope system — per-segment curves, the sustain-less AHD, and a shared overlay for all three envelopes

Trigger's fade pair folds into the AHD (and goes live); the release anchors right;
Preserve rings its synthetic tail out instead of cutting it. Payload v10.

tests/test_theme.cpp

@@ -121,6 +121,27 @@ static void testSecondaryTertiaryAreDistinguishable() {
     CHECK(delta >= 60);
 }
 
+// The instrument's envelope overlay is traced OVER the waveform, which draws in the primary
+// accent — an accent-on-accent pair no floor covers, since neither is a surface. It moved from
+// the secondary to the tertiary for exactly this reason, so the pair is pinned two ways: the
+// tertiary must separate from the primary MORE than the secondary did (the measurable half of
+// the move), and the separation is a hue one, since two pastels sit close in luminance by
+// construction. Whether the result reads clearly is a perceptual call, not this test's.
+static void testOverlayAccentSeparatesFromTheWaveformAccent() {
+    const KitColor wave = roleColor(Role::AccentPrimary);
+    const KitColor overlay = roleColor(Role::AccentTertiary);
+    const KitColor prior = roleColor(Role::AccentSecondary);
+    CHECK(contrastRatio(overlay, wave) > contrastRatio(prior, wave));
+    // Hue divergence against the waveform: the waveform's green dominates its red, the
+    // overlay's red dominates its green — opposite balances, not two shades of one.
+    CHECK(wave.g > wave.r);
+    CHECK(overlay.r > overlay.g);
+    const int delta = std::abs(int(wave.r) - int(overlay.r)) +
+                      std::abs(int(wave.g) - int(overlay.g)) +
+                      std::abs(int(wave.b) - int(overlay.b)) ;
+    CHECK(delta >= 60);
+}
+
 static void testWarnClearsStateFloorOnBackground() {
     // warn (destructive) must be unmistakable -> clears the state floor on the base.
     CHECK(contrastRatio(roleColor(Role::Warn), roleColor(Role::BgBase))
@@ -237,6 +258,7 @@ int main() {
     testTextOnPastelFillClearsBodyFloor();
     testTextOnHoverSurfaceClearsFloor();
     testSecondaryTertiaryAreDistinguishable();
+    testOverlayAccentSeparatesFromTheWaveformAccent();
     testWarnClearsStateFloorOnBackground();
     testLabelOnActiveSurfaceClearsFloor();
     testRolesAreDistinctAndElevationMonotonic();