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.h

@@ -27,15 +27,19 @@ inline constexpr double kLiveRampSeconds = 0.020;
 // morphLaw rides inside filterSettings only because it is cheaper to carry the whole struct to
 // the filter's prepare() than to splice it back; it changes only across a reload, which
 // republishes this block, so the two can never disagree.
+// Each envelope carries BOTH mode shapes: which one a voice applies is fixed at note-on by
+// its play mode, so publishing both keeps the block one shape regardless of mode. The pitch
+// envelope's `enabled` rides along inside its params only because the struct is carried whole;
+// PitchEnvelope ignores it, since a toggle travels by reload.
 struct LiveValues {
     filter::FilterSettings filterSettings{};
     double       filterModAmount = 0.0;
     double       filterKeyTrack = 0.0;
     AdsrParams   filterEnv{};
+    AhdParams    filterAhd{};
     AdsrParams   adsr{};
-    std::int64_t pitchEnvAttackFrames = 0;
-    std::int64_t pitchEnvDecayFrames = 0;
-    double       pitchEnvPeakSemitones = 0.0;
+    AhdParams    ampAhd{};
+    PitchEnvParams pitchEnv{};
 };
 
 // The seqlock copies the block as raw bytes, which is only defensible for a plain value type.