[MEDIUM] macOS canvas redraw-gating change has no test coverage

_draw now skips figure.draw whenever figure.stale is false, and draw/draw_idle only call invalidate, but no test in the repository reaches these paths because the suite runs on Agg and never instantiates the macOS canvas. A regression such as the blit pattern (canvas.draw() followed by copy_from_bbox reading an unrendered buffer, since draw() no longer renders synchronously) or a draw_idle() on a non-stale figure no longer forcing a repaint would ship silently.

SUSPECTED: the diff adds no tests, and the changed code requires the macOS-only _macosx extension that the Agg-based suite never imports. A macOS-runner or stubbed-canvas test asserting that _draw returns without calling figure.draw when figure.stale is false, and that draw() leaves the renderer unrendered until the event loop runs, would settle it.