docs(envelope): add Trigger frames<->fraction seam note + zero-delta guard on fade nodes

src/vst/envelope_edit.cpp

@@ -93,7 +93,16 @@ AmpEnvelope resolveNodeDrag(const AmpEnvelope& grabEnv, EnvNode node, const Rect
         // --- Trigger: fades + length are FRACTIONS. X pixels convert to a fraction of the PLAYED
         //     span (fades) or the whole sample (length). Monotonic: fadeIn + fadeOut <= 1 so the
         //     two fade nodes never cross (each clamps against the other), and length in [0, max].
+        //
+        //     TRIGGER SEAM — CONVERSION REQUIRED ON BOTH PATHS (Wave 2 shell author, read this):
+        //     fadeInFraction/fadeOutFraction in AmpEnvelope are fractions of the played span.
+        //     TriggerParams (sampler_core.h) stores the corresponding values as SOURCE FRAMES
+        //     (fadeInFrames/fadeOutFrames, int64_t). The shell owes a converter on BOTH directions:
+        //       pack   (draw):   fadeInFrames/fadeOutFrames -> fraction (needs frameCount + rate)
+        //       unpack (commit): fraction -> fadeInFrames/fadeOutFrames (same inputs)
+        //     See the TRIGGER SEAM note on AmpEnvelope in envelope_overlay.h for the formula.
         case EnvNode::FadeInEnd: {
+            if (dxPixels == 0) break;  // zero-motion grab: no param change, no division
             const double playSeconds = std::max(0.0, grabEnv.lengthFraction) * totalSeconds;
             const double dFrac = playSeconds > 0.0 ? dSec / playSeconds : 0.0;
             const double hi = std::min(bounds.maxFadeInFraction,
@@ -102,6 +111,7 @@ AmpEnvelope resolveNodeDrag(const AmpEnvelope& grabEnv, EnvNode node, const Rect
             break;
         }
         case EnvNode::FadeOutStart: {
+            if (dxPixels == 0) break;  // zero-motion grab: no param change, no division
             // FadeOutStart sits at (1 - fadeOut) of the played span; dragging it LEFT (negative dx)
             // lengthens the fade-out. So the fade-out fraction moves OPPOSITE the pixel delta.
             const double playSeconds = std::max(0.0, grabEnv.lengthFraction) * totalSeconds;