fix(input): cursors follow the pointer while dragging and snap on drop

tests/test_plot.cpp

@@ -552,6 +552,43 @@ PLOT_TEST(a_cursor_owns_only_the_coordinate_and_snap_of_its_orientation) {
   PLOT_CHECK_NEAR(p.cursor(marker).x, 12.0, 1e-12);
 }
 
+/// The setter's two acts, and the explicit commit between them: `CursorPlacement::Free` stores
+/// the coordinate verbatim even on a `NearestSample` cursor -- the value a drag writes under the
+/// pointer -- and `snap_cursor()` is what later puts it on the grid. The default is the other
+/// act (snapping now), so a programmatic placement is unchanged; a ruler's commit uses its own
+/// rule, which is what makes the two orientations snap at the same moment of a gesture.
+PLOT_TEST(free_placement_defers_the_snap_until_snap_cursor) {
+  plot::data::ArraySeries s(ramp(100), 0.0, 1.0);  // samples at x = 0, 1, 2, ...
+  plot::Plot p;
+  p.set_viewport({0.0f, 0.0f, 800.0f, 400.0f});
+  const u32 ax = p.add_x_axis({"t", {0.0, 99.0}, false, plot::AxisPlace::Bottom, false});
+  const u32 ay = p.add_y_axis({"v", {0.0, 99.0}, false, plot::AxisPlace::Left, false});
+  p.add_trace({&s, plot::Color::hex(0xFFFFFF), 1.0f, ax, ay});
+  const u32 marker = p.add_cursor(12.0, plot::Color::hex(0xFFB020));
+  p.set_cursor_snap(marker, plot::SnapMode::NearestSample);
+  PLOT_CHECK_NEAR(p.cursor(marker).x, 12.0, 1e-12);
+
+  // Free: the value is kept, and it is on no sample of the grid.
+  p.set_cursor_x(marker, 12.6, plot::CursorPlacement::Free);
+  PLOT_CHECK_NEAR(p.cursor(marker).x, 12.6, 1e-12);
+
+  // The commit applies the cursor's own rule -- the nearest sample -- exactly once.
+  p.snap_cursor(marker);
+  PLOT_CHECK_NEAR(p.cursor(marker).x, 13.0, 1e-12);
+
+  // The default placement snaps at once, so the free act is an opt-in, not a new default.
+  p.set_cursor_x(marker, 20.6);
+  PLOT_CHECK_NEAR(p.cursor(marker).x, 21.0, 1e-12);
+
+  // A ruler's commit is its own rule: a Free y is kept, and `snap_cursor()` is a no-op on a
+  // `Free` ruler -- there is no sample grid a y coordinate could land on.
+  const u32 ruler = p.add_h_ruler(50.0, plot::Color::hex(0x35D0A8));
+  p.set_cursor_y(ruler, 33.37, plot::CursorPlacement::Free);
+  PLOT_CHECK_NEAR(p.cursor(ruler).y, 33.37, 1e-12);
+  p.snap_cursor(ruler);
+  PLOT_CHECK_NEAR(p.cursor(ruler).y, 33.37, 1e-12);
+}
+
 /// `NearestTick` lands on a tick of the ruler's own y axis, and `Free` stores the value it was
 /// handed -- so the two modes are told apart by what the store holds, not by a flag. The tick set
 /// is the one `layout()` refreshed: ticks are generated there and nowhere else, which is why a