Animations: blend when a Play animation takes over (#106)

Animations.md

@@ -78,6 +78,16 @@ with the object it played on as `object`. It is heard by the behaviors of that o
 subtree — the object itself or one placed below the owner next to the behavior — and by a behavior
 without an owner for every object; its *Animation* setting narrows it to one.
 
+**Blend.** `Play animation` has a *blend (frames)* port, 0 by default. Above 0, an animation that
+takes over fades in over that many of its own frames: numbers and colours mix linearly from what the
+property held into the animation's value; a texture, a boolean, a text or a choice switches at once.
+"What the property held" is live where an animation started earlier writes there in the same frame —
+the walk it takes over from keeps moving under the fade — and otherwise the value when the blend
+began, be it the template's or one a stopped animation left. A third start during a blend fades from
+the value already mixed, so nothing jumps. The blend starts only when the animation takes over: played
+again every frame while it already leads, it keeps the blend it has. Winning back one that still runs
+blends again with the frames given then; the tab's preview plays one animation and has no blend.
+
 **It runs in play only.** The Scene and Container tabs show the objects as stored; the edit mode
 has no clock. The fastest way in is right-clicking a texture → **Create animation from texture**,
 which makes a sprite of the texture and an animation over it with a `texture` row keyed frame by