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_controls.cpp

@@ -185,7 +185,6 @@ void ReaSamplerEditor::applyControl(int id, PlaySeconds& play, double value,
                 case ParamControl::kPitchEnvMode:  play.pitchSpline.mode = m; break;
                 default:                           play.filterSpline.mode = m; break;
             }
-            if (splineActive(play)) play.playMode = PlayMode::Trigger;
             break;
         }
         case ParamControl::kPitchEngine:
@@ -271,6 +270,12 @@ void ReaSamplerEditor::applyControl(int id, PlaySeconds& play, double value,
             play.filter.trigEnv.decayCurve = util::curveFromKnobNorm(value); break;
         default: break;
     }
+    // ONE normalization point for every control that can flip splineActive — a mode toggle
+    // (above) or an enable toggle (kPitchEnvEnable/kFilterEnable), whose enabling can make an
+    // already-Spline pitch/filter envelope newly active. Applying it once here, rather than at
+    // each site that could cause the flip, is what keeps a future such control from reopening
+    // the same hole.
+    if (splineActive(play)) play.playMode = PlayMode::Trigger;
 }
 
 double ReaSamplerEditor::liveSampleRate() const {