render_mpl and render_animation ignore ViewState.zoom

[bjmorgan]

Summary

The auto-fit viewport path in _draw_scene computes axis limits from the projected bounding box of all atoms. Since ViewState.zoom scales the projected coordinates, a larger zoom produces a proportionally larger bounding box -- the viewport expands to fit and the zoom has no visible effect.

This means:

• Setting view.zoom in the interactive viewer, then calling render_mpl() produces the same output regardless of zoom level.
• The intended workflow of "dial in the view interactively, then render" does not work for zoom.
• render_animation inherits the same issue.

Rotation, pan (view.centre), and perspective are all respected correctly. Only zoom is broken in the auto-fit path.

Expected behaviour

zoom > 1 should show a cropped/zoomed-in view of the scene. zoom < 1 should show the scene with more padding around it.

Root cause

_draw_scene in painter.py (auto-fit path, lines ~800-805) computes pad_x and pad_y from the projected coordinates, which already have zoom applied via ViewState.project(). The zoom scales both the coordinates and the bounding box equally, so they cancel out.