Notify Slider observers before requesting a draw in set_val - #32535

What

Fixes #32526.

Slider.set_val called canvas.draw_idle() before notifying 'changed'
observers, so a callback that updates other artists caused two renders per
event — the first drawn with the previous data. In the reporter's app (four
panels redrawn per slider event) one event cost 97 ms, 49 ms of it the wasted
stale render.

Changes

• lib/matplotlib/widgets.py — Slider.set_val: notify 'changed'
  observers first, then request the draw when drawon is set. self.val is
  assigned before notification so observers see the new value. A callback's
  own draw_idle() now coalesces with the widget's single draw instead of
  producing a second, stale one. (RangeSlider.set_val has the same shape
  but is left untouched — out of scope of the reported issue.)
• lib/matplotlib/tests/test_widgets.py — new regression test
  test_slider_set_val_notifies_before_draw: wraps canvas.draw_idle and
  asserts the observer ran before set_val requested its draw.

Verification

• Reproduced on unpatched main with the issue's scenario (Agg backend):
  the draw was requested with 0 observer events fired; after the fix the
  single draw is requested after the observer ran.
• New test fails on unpatched code (draw_calls == [0]), passes with the
  fix (draw_calls == [1]).
• test_widgets.py in full: 135 passed, 1 skipped (all 11 slider cases
  pass; 2 image-comparison tests fail only in wheel installs that lack
  baseline images — unrelated to this change; they pass when run from the
  repo tree).
• ruff check (repo-pinned v0.15.20): clean on both changed files.

Known limitations

• The full repo test suite was not run; the change is confined to
  Slider.set_val, and test_widgets.py is fully green.

AI Disclosure

AI tools assisted in the development of this patch; all changes were reviewed and tested by me, and I take responsibility for them.