fix(trigger-seam): thread startFrame into both envelope converters; extract pure triggerPlayLength module + tests

src/vst/reasampler_editor.h

@@ -281,12 +281,16 @@ private:
 
     // PACK (draw): zone play params -> AmpEnvelope. Copies AHDSR seconds directly; derives the
     // Trigger fade fractions from the source-frame fades over the played span.
-    AmpEnvelope packEnvelope(const ZonePlaySeconds& play, std::int64_t frames) const;
+    // `startFrame` is the zone's effective start point (zone.startPoint.value_or(0)).
+    AmpEnvelope packEnvelope(const ZonePlaySeconds& play, std::int64_t frames,
+                             std::int64_t startFrame) const;
 
     // UNPACK (commit): an edited AmpEnvelope -> the zone's play params. Copies AHDSR seconds
     // directly; converts the Trigger fade fractions back to source frames over the played span.
+    // `startFrame` is the zone's effective start point (zone.startPoint.value_or(0)).
     // Mutates `play` in place; only the mode-relevant fields are written.
-    void unpackEnvelope(const AmpEnvelope& env, std::int64_t frames, ZonePlaySeconds& play) const;
+    void unpackEnvelope(const AmpEnvelope& env, std::int64_t frames, std::int64_t startFrame,
+                        ZonePlaySeconds& play) const;
 
     // The clamp bounds envelope_edit uses, matching the control-panel sliders' own domains (so a
     // node drag can never produce a param a slider couldn't — the S-VIEW-F2 invariant).
@@ -385,6 +389,7 @@ private:
     WaveMarker waveMarker_ = WaveMarker::kStart;
     SetupMarkers dragStartMarkers_;
     std::int64_t dragSampleFrames_ = 0;   // decoded length of the sample under the drag
+    std::int64_t dragStartFrame_  = 0;   // zone startPoint at grab time (0 if absent); for env-node drag
 
     // S12 scrollbar-thumb drag: the offset held at grab time (the pixel-delta resolver shifts
     // from it). S12/S15/S16 param-slider drag: which control id + the panel it lives in (the