instrument: fix AHD node-tracking/tie-break/live-latch defects and close staged-envelope-curve test gaps

src/core/instrument/engine/play_params.h

@@ -55,8 +55,8 @@ struct AdsrParams {
 
 // Attack -> Hold -> Decay over a bounded span: the shape every SUSTAIN-LESS envelope takes
 // (the Trigger amp, the Trigger filter envelope, the pitch envelope). Hold is a FRACTION of
-// the span left after attack and decay, never a time of its own — that is what makes
-// A + H + D <= span structural rather than clamped (see fitAhd in envelopes.h).
+// the span left after attack and decay, never a time of its own — fitAhd (envelopes.h) owns
+// why a fraction, not a time.
 struct AhdParams {
     std::int64_t attackFrames = 0;
     std::int64_t decayFrames  = 0;