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.

docs/VERIFICATION.md

@@ -1,6 +1,6 @@
 # DAW verification — post-1.0 work on `dev`
 
-Checks for Θ, Ξ, Ψ, Ε, Ρ, and Γ work that no unit test can close. Build **Release**, install into
+Checks for Θ, Ξ, Ψ, Ε, Ρ, Γ, and Ω work that no unit test can close. Build **Release**, install into
 `UserPlugins/`, restart REAPER. Panel tail toggle = **None**, project rate 48000, unless a check says otherwise.
 
 ## Precision invariants
@@ -167,6 +167,20 @@ Checks for Θ, Ξ, Ψ, Ε, Ρ, and Γ work that no unit test can close. Build **
 - [ ] Open a project saved before the zone retirement — the instance reopens on its first zone and sounds the same (`docs/COMPLETED.md` §"Θ-W1-T1")
 - [ ] Such an instance with implicit channel mode + a stereo capture reopens **Stereo** — confirm that is acceptable by ear (`docs/COMPLETED.md` §"Θ-W1-T1")
 
+## Waveform marker zero-crossing snap (Ω-W2-T5)
+
+- [ ] `[verify — DAW]` Drag a Gate loop mark on a single-cycle capture — the felt radius the mark
+  may be pulled to reach a crossing genuinely reads as imperceptible, i.e. a displacement inside
+  the pointer's own grab neighbourhood is not one the eye catches (`src/core/instrument/ui/waveform_view.h`'s
+  `kZeroCrossingSnapPx` comment)
+- [ ] `[verify — DAW]` Hold Ctrl, then drag a waveform marker (start/loop start/loop end) off a
+  zero crossing — confirm it lands exactly where dropped, with no snap, in a live editor window
+  (`editor_input_waveform.cpp`'s `dragWaveform`, `GetKeyState`-driven, no CI test target)
+- [ ] `[verify — DAW]` Press and hold Ctrl BEFORE mouse-down on a mark that coincides with a
+  contour node or another mark's cap — confirm the mark is grabbed (not the node dragged, not a
+  hard/smooth toggle fired), matching the fix in `resolveWaveformClaim`
+  (`src/core/instrument/ui/spline_edit.cpp`)
+
 ## Look and feel
 
 - [ ] Sign off by eye in a live editor window: knob arcs, needles, envelope splines, waveform outline (`docs/COMPLETED.md` §"Θ-W7-T1", `docs/COMPLETED.md` §"Θ-W6-T1" — "Antialiasing pass")