Ω-W1-T5: fix two stale comments in test_waveform_view.cpp (retired proportional-round model; sweep's structural-not-behavioural role)

tests/test_waveform_view.cpp

@@ -244,13 +244,13 @@ static void testResolveDragFrameClamps() {
     CHECK(resolveDragFrame(ov, 1000, 950, 500) == 1000);   // clamp high (== frameCount)
 }
 
-static void testResolveDragFrameRounds() {
-    // 500px area over 1000 frames -> 2 frames/px. A +3px drag -> round(6.0)=6; the rounding is
-    // at the frame centre. Use a scale where a fractional result appears.
-    const OverlayArea ov = overlayOf(Rect::ltrb(0, 0, 300, 60));  // 1000 frames / 300px = 3.33 frames/px
-    // +3px -> 3*1000/300 = 10.0 -> 10 frames.
+static void testResolveDragFrameTruncatesAtFractionalScale() {
+    // 300px area over 1000 frames -> 3.33 frames/px, so frameToX(start) -> +dx -> xToFrame
+    // lands on the column's truncating partition rather than a whole multiple of dx.
+    const OverlayArea ov = overlayOf(Rect::ltrb(0, 0, 300, 60));
+    // frameToX(100) = 30; xToFrame(30 + 3) = 33*1000/300 = 110.0 -> 110 frames.
     CHECK(resolveDragFrame(ov, 1000, 100, 3) == 110);
-    // +1px -> 1000/300 = 3.33 -> rounds to 3.
+    // frameToX(100) = 30; xToFrame(30 + 1) = 31*1000/300 = 103.33 -> truncates to 103.
     CHECK(resolveDragFrame(ov, 1000, 100, 1) == 103);
 }
 
@@ -279,8 +279,10 @@ static void testResolveDragFrameLandsOnCursorColumn() {
     const std::int64_t cursorFrame = xToFrame(ov, frameCount, grabX + dx);
     CHECK(cursorFrame == 11);                              // the contract's own derivation
     CHECK(resolveDragFrame(ov, frameCount, startFrame, dx) == cursorFrame);
-    // General form, swept across both frames<columns and frames>columns: the resolved frame's
-    // OWN column (frameToX) must contain the cursor pixel the drag actually landed on.
+    // Structural coverage, not behavioural: this sweep asserts resolveDragFrame's own definition
+    // (frameToX then xToFrame) and cannot fail while it calls those two functions. Its value is
+    // pinning that there is no second, independent mapping hiding in some frames<columns or
+    // frames>columns corner — the single behavioural anchor is the literal 11 above.
     const std::int64_t counts[] = {5, 37, 251, 9973};
     const int deltas[] = {-97, -3, -1, 1, 3, 97};
     for (std::int64_t n : counts) {
@@ -815,7 +817,7 @@ int main() {
 
     testResolveDragFrameShift();
     testResolveDragFrameClamps();
-    testResolveDragFrameRounds();
+    testResolveDragFrameTruncatesAtFractionalScale();
     testResolveDragFrameDegenerate();
     testResolveDragFrameLandsOnCursorColumn();