docs(ledgers): record source-backed findings on the figures of Iapetus and Ceres and the limbs of the Galilean and Uranian moons

[alowpoly]

Findings from a source sweep, written into the ledgers; no globe changes.

• Iapetus and Ceres are drawn as spheres but are flattened by 4.5% and 7.5% (NAIF's pck00010.tpc; the Dawn project's kernel). I tried the spheroids and withdrew them: the ledgers say which three things in the shared sphere geometry must change first.
• Europa: a 1983 NASA summary (McEwen and Soderblom, 44 image pairs) and the 2021 paper's own 10° image both give a flood-lit limb near 0.62 of the centre; we draw 0.86.
• Ganymede, Callisto: the same summary independently finds lunar-like scattering. Io: it prints a per-color limb law, recorded as an alternative.
• Titania: its law, checked against five calibrated Voyager frames from 0.8° to 70° phase, matches the resolved limb to 0.06. Oberon and Umbriel carry the same check, less conclusive.

See the Europa and Iapetus ledgers. One stale sentence about lighting in the Iapetus README is corrected.

Limits: text only. The frame checks fit one weight to a whole lit disc, so uneven albedo biases them; Umbriel's includes the bright annulus its authors left out.

Clean: the ledger test and the repository text checks.

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