feat(saturn): a limb law per display channel, no dashes along the limb, and Cassini's 2011 storm map as a dataset

[alowpoly]

Saturn's limb was olive with dark dashes along it, and its surface is softer than the other planets'. Now the limb pales without turning green, the dashes are gone, and a sharper Cassini map is a new dataset. The default surface stays Hubble's: its softness is the 1,800-pixel source. Bodies: Saturn. Shared: the observed-surfaces lane. Details: Saturn README.

• Limb color: each display channel takes Hubble's per-filter exponents weighted by the light it shows (0.798, 0.698, 0.728; was 0.80, 0.65, 0.86). Green/red at the edge: 1.02 → 0.89.
• Dashes: under flood light the limb law was held on one angle only, so scattered outer texels fell to black. Dark dips along 32° of limb: 29 → 3 (Jupiter 0).
• Cassini 2011: the PDS contrast-enhanced color cube (in the app). Its header states rows and columns backwards; a raw frame fixes them (correlation 0.62, mirrored 0.23).
• Bake: Saturn could not be baked on main since refactor(runtime): no CSS custom property where a body is drawn; billboard, light roll, meshes and step names are direct writes and records #1323; the layout reader now reads the plain pixel box.

Limits: Chrome only. With shadows on, the lit limb keeps fainter dashes (32 → 19 dips). Cassini's color is the archive's stretch; 17.7 % of the map is blank.

• Baked saturn (twice: the first bake's capture exposed the clamp), published to R2, inventory committed.

Clean: observed-surfaces, material-composition, layered-oblate, photometry, display-limb, source-record and runtime-package tests; bake typecheck; ESLint; check-assets-published 179 of 179.

PR rules: titles, PDS4 provenance and ISO plain language.