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.

tests/test_spline_edit.cpp

@@ -280,15 +280,27 @@ static void testAMissedCandidateNeverWinsOnADegenerateZeroArea() {
           WaveformClaimant::kTab);
 }
 
-// A control-click has no tab/marker meaning (only the node's hard/smooth toggle answers it), so
-// it resolves to the node whenever the node is in the running, even where a plain left-click at
-// the same pixel would hand the tab or marker the win on area alone.
-static void testControlClickAlwaysTakesTheNodeOverASmallerTabOrMarker() {
+// A control-click has no tab/marker meaning (only the node's hard/smooth toggle answers it), but
+// that must not let Ctrl steal a mark grab out from under the cursor: when a cap or column is
+// ALSO in the running, control-click defers to the ordinary smallest-area arbitration exactly
+// like a plain left-click would, so pressing Ctrl before or after the button gives the same
+// answer. Only with no cap/column in the running at all does control-click claim the node
+// outright regardless of area.
+static void testControlClickDefersToACoincidentCapOrColumnLikeAPlainClick() {
     const WaveformClaim node{true, kNodeArea};
     const WaveformClaim smallerTab{true, 50};  // would beat the node on a plain left-click
     CHECK(resolveWaveformClaim(node, smallerTab, WaveformClaim{}, SplineGesture::kLeft) ==
           WaveformClaimant::kTab);
+    // Ctrl pressed before the click must not out-rank the cap that a plain click already gives
+    // the win — the exact regression this pins.
     CHECK(resolveWaveformClaim(node, smallerTab, WaveformClaim{}, SplineGesture::kControlLeft) ==
+          WaveformClaimant::kTab);
+    const WaveformClaim smallerMarker{true, 80};  // still smaller than the node, no tab present
+    CHECK(resolveWaveformClaim(node, WaveformClaim{}, smallerMarker,
+                               SplineGesture::kControlLeft) == WaveformClaimant::kMarker);
+    // No cap or column at all: control-click still claims the node outright, regardless of area
+    // — there is nothing else for it to defer to.
+    CHECK(resolveWaveformClaim(node, WaveformClaim{}, WaveformClaim{}, SplineGesture::kControlLeft) ==
           WaveformClaimant::kNode);
     // No node in the running: control-click has nothing to fall back to, so the tab still wins.
     CHECK(resolveWaveformClaim(WaveformClaim{}, smallerTab, WaveformClaim{},
@@ -311,7 +323,7 @@ int main() {
     testTabWinsAGenuineTabVersusMarkerTie();
     testNoHitAnywhereFallsThroughToNone();
     testAMissedCandidateNeverWinsOnADegenerateZeroArea();
-    testControlClickAlwaysTakesTheNodeOverASmallerTabOrMarker();
+    testControlClickDefersToACoincidentCapOrColumnLikeAPlainClick();
 
     if (g_fail == 0) std::printf("spline_edit: all tests passed\n");
     return g_fail == 0 ? 0 : 1;