The Dart-side predictive curve plays a sampled LUT to lead the system IME
animation by `_headStart`, so the composer can render where the IME will
be by the time the Flutter frame paints. When the LUT hasn't yet adapted
to the actual device curve, the prediction runs above the IME's real
position and the composer overshoots — quite visibly in debug mode where
Flutter's lower frame rate amplifies per-frame divergence, milder in
release where the rate is higher and learning converges faster.
Cap each per-frame `h` against the system-reported value (which is
per-vsync fresh on Android R+ via WindowInsetsAnimation.onProgress and
iOS via CADisplayLink). The clamp is a no-op on Android pre-R because
the native value snaps to target up-front, so the Huawei pre-R fallback
keeps animating via the LUT alone. Net effect on adapted devices: the
predictive lead is sacrificed for a per-frame "track the IME exactly"
guarantee — no overshoot regardless of LUT freshness.
Also narrow the previous-commit's `scheduleFrame` after notifyListeners
to fire only at the close-edge (`!isIOS && _height == 0`). Mid-animation
the steady-state pump or curveActive condition already schedules; only
the down-to-zero transition needs the explicit flush.
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