fix(instrument): stop Ctrl-before-click stealing a waveform mark grab from a coincident node

Ctrl held before mouse-down forced the node/toggle win over a smaller cap or
column regardless of area; now it defers to the ordinary smallest-area
arbitration like a plain click. Also amends product docs, VERIFICATION.md, and
adds sparse-material/narrow-overlay test fixtures.

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

@@ -286,10 +286,11 @@ void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
     // 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.
+    // 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 && !dragModifiers().ctrl) {
-        newFrame = snapToZeroCrossing(pcm.data(), static_cast<std::int64_t>(pcm.size()), newFrame,
+        newFrame = snapToZeroCrossing(pcm.data(), frames, newFrame,
                                       zeroCrossingSnapFrames(overlay, frames));
     }