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_controller.cpp

@@ -322,7 +322,12 @@ PLOT_TEST(non_finite_wheel_does_not_break_a_range) {
   PLOT_CHECK(std::fabs(p.x_range(ax).min) < 1e6 && std::fabs(p.x_range(ax).max) < 1e6);
   PLOT_CHECK(p.x_range(ax).span() < span_before);
 
-  // A non-finite amount is no zoom at all.
+  // A non-finite amount is no zoom at all -- and, just as important, it must not
+  // touch the axes. A *zeroed* amount would be a zero-notch zoom (a factor of
+  // exactly one): the range would survive, but the range setters it runs through
+  // clear auto scale, so one garbage wheel would quietly disarm auto-fit.
+  p.set_x_auto_scale(ax, true);
+  p.set_y_auto_scale(ay, true);
   const plot::Range xr = p.x_range(ax);
   const plot::Range yr = p.y_range(ay);
   c.on_wheel({400.0f, 200.0f, 0.0f, nan, 0});
@@ -330,6 +335,8 @@ PLOT_TEST(non_finite_wheel_does_not_break_a_range) {
   PLOT_CHECK_NEAR(p.x_range(ax).min, xr.min, 1e-12);
   PLOT_CHECK_NEAR(p.x_range(ax).max, xr.max, 1e-12);
   PLOT_CHECK_NEAR(p.y_range(ay).min, yr.min, 1e-12);
+  PLOT_CHECK(p.x_axis(ax).auto_scale);
+  PLOT_CHECK(p.y_axis(ay).auto_scale);
 }
 
 PLOT_TEST(key_f_auto_fits_traces) {