feat: add as_points tutorial

Show as_points on render_shapes and render_labels using a real Xenium
breast-cancer section (167780 cells). Covers what it draws (one centroid
dot per element), the appearance controls that still apply (color, size,
cmap, palette), what it ignores (outline_*, shape, contour_px), the
matplotlib and datashader backends with the 50000-centroid switch, and
the measured speed trade-off: a large win for shapes (up to 16x), about
neutral for labels (a mask already renders as a fast raster).

Segmentation masks are derived from the real cell_boundaries via
rasterize(return_regions_as_labels=True), since Xenium rep1 ships
outlines as shapes with no native labels element.

The 3.4 GB download makes the notebook too heavy for CI, so it is
excluded from execute.yaml and its committed outputs are trusted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: timtreis