fix(input): refuse a non-finite wheel amount instead of zeroing it

Zeroing an unusable wheel amount turned it into a zero-notch zoom: the factor was
exactly one, the range did not change, but the zoom body still ran through
set_x_range/set_y_range, which clear auto_scale unconditionally. One garbage wheel
therefore disarmed auto-fit on every axis. on_wheel now rejects an unusable amount
before the event can mutate anything, keeping the position fallback.

The tests that should have caught it are strengthened: the wheel case now asserts
auto scale is still armed (it was passing either way), and the invalid-position
case checks a pan measured from the last real position with the drag alive, rather
than a range nothing could have moved.

tests/test_imgui_widget.cpp

@@ -344,10 +344,24 @@ PLOT_TEST(imgui_widget_invalid_position_does_not_move_the_range) {
   f.frame(ImVec2(-kMaxFloat, -kMaxFloat), false);
   PLOT_CHECK(!f.panning());
 
-  // And a real position after the sentinel is not differenced against the sentinel.
+  // A drag held across a sentinel, then a real position again. The movement must be
+  // measured from the last *real* position -- 100 px from (200, 200) to (300, 300) --
+  // and not from the sentinel. This is asserted with the drag alive on purpose:
+  // nothing can move a range once the button is up, so the same check after the
+  // release could not fail and would prove nothing.
+  f.frame(ImVec2(200.0f, 200.0f), false);
+  f.frame(ImVec2(200.0f, 200.0f), true);
+  const plot::Range second_press = f.x_range();
+  f.frame(ImVec2(-kMaxFloat, -kMaxFloat), true);
+  PLOT_CHECK(f.panning());
+  const double axes_w = static_cast<double>(std::as_const(f.plot).layout().axes.w);
+  const double expected_shift = 100.0 / axes_w * second_press.span();
+  f.frame(ImVec2(300.0f, 300.0f), true);
+  PLOT_CHECK(f.panning());
+  PLOT_CHECK(std::isfinite(f.x_range().min));
+  PLOT_CHECK_NEAR(f.x_range().min, second_press.min - expected_shift, 0.05 * expected_shift);
   f.frame(ImVec2(300.0f, 300.0f), false);
-  PLOT_CHECK_NEAR(f.x_range().min, pressed.min, 1e-9);
-  PLOT_CHECK_NEAR(f.x_range().max, pressed.max, 1e-9);
+  PLOT_CHECK(!f.panning());
 }
 
 PLOT_TEST(imgui_widget_paints_the_whole_child_when_nothing_clips_it) {