feat(stars): add the central stars of the Ring, Owl, Helix and Southern Ring nebulae

[alowpoly]

The Ring, the Owl, the Helix and the Southern Ring each get their central star as an object inside the nebula: WD 1851+329, WD 1111+552, WD 2226-210 and the Southern Ring's faint white dwarf. A zoom in on each nebula goes on into its star, and the star's page keeps the nebula's walls around it. Each star is a sphere of the size that radiates its published luminosity at its published temperature: 93,000 to 112,000 K, two and a half to four times Earth's size.

The other three strips are in their READMEs.

Limits: no paper prints these radii; they follow from luminosity and temperature. Masses are unmeasured. Three stars are dimmed toward the edge by the nearest tabulated white-dwarf model (Claret et al. 2020, pure hydrogen, up to 100,000 K), not a measurement. The Southern Ring's star is a flat disc: nothing is published about its atmosphere; its values are those a model of the nebula needs, and its bright companion HD 87892 is not drawn. Tooling: the generator and the color reader now take temperatures up to 250,000 K. Headless Chrome only.

Prepared assets

• Baked the four stars.
• Published the bytes.
• Committed the refreshed inventory.json.

Clean: generator and renderer tests, typecheck, lint and the repository checks.

PR rules.