fix: close round-2 review findings — Critical silent-note bug plus majors/minors

Fixes the 2-point spline-EG early-free bug causing silent fade-ins, the pitch/filter enable-toggle Trigger-forcing hole, the contour-node/marker pixel shadow, missing deck-residue test coverage, and stale comments in knob_deck and spline_edit.

src/shell/instrument/editor_models.cpp

@@ -69,10 +69,14 @@ StageEnvelope ReaSamplerEditor::packEnvelope(OverlayEnv which, const PlaySeconds
     // post-start span, since it keeps running after a Trigger one-shot's amplitude has ended.
     const std::int64_t playLen =
         triggerPlayLength(play.trigger.lengthFraction, frames, startFrame);
-    const double playSpan = rate > 0.0 ? static_cast<double>(playLen) / rate : 0.0;
     const double fullSpan =
         rate > 0.0 ? static_cast<double>((std::max)(std::int64_t{0}, frames - startFrame)) / rate
                    : 0.0;
+    // Voice::start makes kTrigLength inert while any spline EG is active (trigSpan == postStart,
+    // not the %-length) — the overlay's amp/filter AHD must read the SAME span the engine plays,
+    // or its drawn shape and node drags cover only a fraction of what the note actually does.
+    const double playSpan =
+        splineActive(play) ? fullSpan : (rate > 0.0 ? static_cast<double>(playLen) / rate : 0.0);
     const bool trigger = (play.playMode == PlayMode::Trigger);
     switch (which) {
         case OverlayEnv::kPitch: