- keyframed state, count, read (time in sequence timebase + mapped
values), add/remove, easing read/write (linear/bezier/hold with
control points), and clear - all undoable with full undo/redo
assertions
- easing set commands are minimal capi-local UndoCommands matching
the app-layer semantics (engine has none of its own); same-time
duplicates are rejected with E_STATE instead of hitting the engine's
debug assert
