Declarative motion for FS.Skia.UI (feature 073). A bounded, additive slice: an author declares — as data against an existing `Scene` — that opacity, an affine transform, and/or color should travel from a start value to a target value over a duration shaped by a named easing curve. Sampling is a **pure** function of an explicit `TimeSpan`, so identical inputs and identical time samples always produce byte-identical output. A deliberate identity-at-rest lowering makes a settled animation byte-identical to the static render of the same widget. The named easing curves. Endpoints are pinned for every case (`Easing.apply e 0.0 = 0.0`, `Easing.apply e 1.0 = 1.0`).