Merge Ω-W2-T5: bound the waveform zero-crossing snap to a pixel radius

Single-cycle loop marks stop teleporting to the one interior crossing. Ctrl on a
marker drag defeats the snap, and no longer loses the grab to a coincident node.

src/shell/instrument/CLAUDE.md

@@ -311,3 +311,11 @@ against a performance budget — they are there because `VoiceEngine::applyLiveT
   "channel identity derives from `app_version` accessors, no scattered `#ifdef`s" —
   `INLINE_UID` needs literal brace-init tokens, so it can't route through a runtime
   string accessor.
+- **Ctrl held during a waveform marker drag defeats the zero-crossing snap outright**
+  (`editor_input_waveform.cpp`'s `dragWaveform`, reading `dragModifiers().ctrl` — the same
+  "get out of the way, I want exactness" role Ctrl plays as the knobs' fine drag). It is
+  undiscoverable by design — no cursor cue, no label — so this note is its only home; the
+  radius itself (`core/instrument/ui/waveform_view`'s `snapToZeroCrossing`) is documented there,
+  not here. `resolveWaveformClaim` (`core/instrument/ui/spline_edit`) is the reason Ctrl held
+  BEFORE mouse-down still reaches the mark rather than stealing a coincident envelope-node grab
+  or hard/smooth toggle.

src/shell/instrument/editor_input_waveform.cpp

@@ -282,13 +282,16 @@ 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. `[verify — DAW]` this
+    // `GetKeyState` read has no test target — nothing in CI observes the defeat actually landing.
     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(), frames, newFrame,
+                                      zeroCrossingSnapFrames(overlay, frames));
     }
 
     // Build the edited marker set from the snapshot, moving only the grabbed marker, then