Session::region_job renders one PixelWindow of the loupe's frame from the source
level that frame size needs, with a key of its own and no cache writes.
ui-egui::region decides when a window is needed (the whole-frame render is
stretched) and which one: the visible pixels plus a margin, snapped to a 256 px
grid so small pans reuse the render, bounded in size, clamped into the frame.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>