fix(input): clamp a dragged ruler on an ordered y range and share the orientation test

tests/test_controller.cpp

@@ -874,6 +874,37 @@ PLOT_TEST(horizontal_ruler_is_grabbed_and_dragged_along_y) {
   PLOT_CHECK(f.plot.cursor(ruler).y > 40.0);
 }
 
+/// An inverted (descending) y range is ordinary -- depth, row index, descending time -- because
+/// `set_y_range()` stores the range verbatim and only the transform widens it. The drag's clamp
+/// must therefore order the bounds: `std::clamp` requires `min <= max`, so a raw
+/// `clamp(y, range.min, range.max)` collapses the ruler onto one end on such an axis (and is
+/// undefined behaviour besides). The ruler has to follow the pointer here, and stay inside the
+/// range.
+PLOT_TEST(horizontal_ruler_follows_the_pointer_on_an_inverted_y_range) {
+  Fixture f;
+  // Descending: the layout maps `range.min` (100) to the axes' bottom and `range.max` (0) to its
+  // top, so a pointer further down is a *larger* y.
+  f.ay = f.plot.add_y_axis({"v", {100.0, 0.0}, false, plot::AxisPlace::Left, false});
+  const u32 ruler = f.plot.add_h_ruler(50.0, plot::Color::hex(0x35D0A8));
+  f.plot.layout();
+  const f32 column = screen_x_of(f.plot, f.ax, 50.0);
+  const f32 row = row_of(f.plot, f.ay, 50.0);
+
+  hover(f.controller, column, row);
+  PLOT_CHECK(f.controller.hovered_cursor() == ruler);
+  press(f.controller, column, row);
+  PLOT_CHECK(f.controller.grabbed_cursor() == ruler);
+
+  const f32 moved = row + 60.0f;
+  move_to(f.controller, column, moved);
+  // Exactly the data under the pointer, not a collapsed bound, and inside the range.
+  PLOT_CHECK_NEAR(f.plot.cursor(ruler).y, data_at_row(f.plot, f.ay, moved), 1e-9);
+  PLOT_CHECK(f.plot.cursor(ruler).y > 50.0);
+  PLOT_CHECK(f.plot.cursor(ruler).y < 100.0);
+  release(f.controller, column, moved);
+  PLOT_CHECK(f.plot.cursor(ruler).y > 50.0);
+}
+
 /// Nearest-first resolution *across* orientations: a pointer within reach of both a marker and a
 /// ruler takes the nearer of the two, so neither widens the other's target -- which is what lets
 /// one hit test serve two instruments. Each press below is 1 px from its winner and 4 px from the