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/map/trigger_seam.cpp

@@ -14,14 +14,4 @@ std::int64_t triggerPlayLength(double lengthFraction,
     return static_cast<std::int64_t>(lengthFraction * static_cast<double>(postStart) + 0.5);
 }
 
-double framesToFadeFraction(std::int64_t fadeFrames, std::int64_t playLength) {
-    if (playLength <= 0) return 0.0;
-    return static_cast<double>(fadeFrames) / static_cast<double>(playLength);
-}
-
-std::int64_t fadeFractionToFrames(double fadeFraction, std::int64_t playLength) {
-    if (playLength <= 0) return 0;
-    return static_cast<std::int64_t>(fadeFraction * static_cast<double>(playLength) + 0.5);
-}
-
 }  // namespace reasampler::instrument::map