fix(instrument): bound the waveform zero-crossing snap to a pixel radius so single-cycle loop marks stop teleporting

Ctrl during a marker drag defeats the snap outright. nearestZeroCrossing keeps its
unbounded contract; snapToZeroCrossing is the decision a drag applies.

src/shell/instrument/editor_input_waveform.cpp

@@ -282,13 +282,15 @@ void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
                                            dragStartMarkers_.crossfade};
     std::int64_t newFrame = resolveDragFrame(overlay, frames, startVals[idx], dx);
 
-    // Snap to the nearest zero crossing in the decoded PCM. Pure over the cached mono
-    // frames — no host types, no file I/O. The crossfade handle is exempt: it sets a fade
-    // LENGTH, and the whole point of the fade is that its edges need no zero crossing.
+    // Snap to a zero crossing in the decoded PCM, but only one within kZeroCrossingSnapPx of
+    // where the mark was dropped. Pure over the cached mono frames — no host types, no file I/O.
+    // The crossfade handle is exempt: it sets a fade LENGTH, and the whole point of the fade is
+    // that its edges need no zero crossing. Ctrl defeats the snap outright, the same "get out of
+    // the way, I want exactness" role it plays as the knobs' fine drag.
     const std::vector<AudioSample>& pcm = monoPcmFor(selectedId_);
-    if (!pcm.empty() && waveMarker_ != WaveMarker::kCrossfade) {
-        newFrame = nearestZeroCrossing(pcm.data(), static_cast<std::int64_t>(pcm.size()),
-                                       newFrame);
+    if (!pcm.empty() && waveMarker_ != WaveMarker::kCrossfade && !dragModifiers().ctrl) {
+        newFrame = snapToZeroCrossing(pcm.data(), static_cast<std::int64_t>(pcm.size()), newFrame,
+                                      zeroCrossingSnapFrames(overlay, frames));
     }
 
     // Build the edited marker set from the snapshot, moving only the grabbed marker, then