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.

src/core/instrument/engine/live_params.cpp

@@ -11,10 +11,10 @@ LiveValues foldLive(const PlayParams& params) {
     v.filterModAmount = params.filter.modAmount;
     v.filterKeyTrack = params.filter.keyTrack;
     v.filterEnv = params.filter.env;
+    v.filterAhd = params.filter.trigEnv;
     v.adsr = params.adsr;
-    v.pitchEnvAttackFrames = params.pitchEnv.attackFrames;
-    v.pitchEnvDecayFrames = params.pitchEnv.decayFrames;
-    v.pitchEnvPeakSemitones = params.pitchEnv.peakSemitones;
+    v.ampAhd = params.trigAhd;
+    v.pitchEnv = params.pitchEnv;
     return v;
 }