iris: drive animation from painter frame time

docs/SCROLL.md

@@ -15,7 +15,7 @@ that struct (`scrollable.rs`); there is exactly one `Flinger` and one
 `DragGesture` implementation in the crate's widgets. A widget with one
 implements `Scrollable`, whose one required pair of methods hands the
 controller back, and gets `scroll`, `fling`, `drag`, `amt`,
-`is_scrolling`, `tick_fling` and the pin as default methods.
+`is_scrolling` and the pin as default methods.
 
 Two widgets have one, and they differ only in how they spend a delta:
 
@@ -30,6 +30,14 @@ Two widgets have one, and they differ only in how they spend a delta:
 Do not give a widget its own fling, scroll amount, or platform wake
 handle, and do not add scrolling methods to the general `Widget` trait.
 
+There is no animation registry or animation callback on `Widget`. The
+platform passes the frame's presentation time through `Painter`; a scrolling
+widget advances its controller at the beginning of `draw` and calls
+`Painter::request_next_frame` while the fling remains active. That request is
+staged until the current retained redraw finishes, then dirties the requesting
+widget for the following frame. Staging matters because the ordinary dirty set
+is drained to completion within one frame.
+
 ## One convention for a delta
 
 **Positive scrolls the reader up or left; negative down or right.** The
@@ -60,14 +68,14 @@ means "the end is not in sight". That last is not a placeholder — a lazy
 layout genuinely cannot say how far its content runs without walking
 there, and `clamp` takes the answer with no branch of its own.
 
-**Why the delta is banked rather than applied where it arrives.** A wheel
-event, a drag frame and a fling tick all land between draws, and none of
-them can know whether there is content to move into. Applying them at the
-layout that follows is also what keeps layout a pure function of the state
-(Iris, 2026-09-08). The visible consequence, and the thing that catches a
+**Why the delta is banked rather than applied where it arrives.** A wheel or
+drag lands between draws, and advancing a fling happens at the top of a draw;
+none of them can know whether there is content to move into until layout runs.
+Applying them in that layout is also what keeps layout a pure function of the
+state (Iris, 2026-09-08). The visible consequence, and the thing that catches a
 test out: **`amt` does not move until the next draw.**
 
-**Which clock a fling is ticked on.** The vsync the frame callback
+**Which clock advances a fling.** The vsync the frame callback
 carries, not `Instant::now()` -- on Android `do_frame`'s
 `frame_time_nanos`, converted through the view's one `DeviceClock`
 (`sense.rs`), which also dates every touch sample, so a fling is advanced