TextComponent glyphs jump up by one ascent when an OpacityEffect starts

[spydon]

Description

The first time an OpacityEffect (or any HasPaint change) updates a TextComponent, the rendered glyphs teleport upward by one font ascent and stay there. Combined with a movement effect this reads as a sudden vertical jump in the middle of an otherwise smooth animation.

Related to #4013 but a distinct defect: that one is about shadow alpha, this one is about glyph positioning.

Cause

TextComponent.updateBounds translates the freshly formatted element down by its ascent:

@internal
void updateBounds() {
  _updateElement();
  final measurements = _textElement.metrics;
  _textElement.translate(0, measurements.ascent);
  size.setValues(measurements.width, measurements.height);
}

but TextComponent.onChanged, which runs on every paint mutation, rebuilds the element without that translation:

@override
void onChanged() {
  _textRenderer = _textRenderer.copyWithPaint(paint) as T;
  _updateElement();
}

So the element created before the first paint change is ascent-translated, and every element created after a paint change is not, shifting the drawn text up by metrics.ascent for the rest of the component's life.

Steps to reproduce

final text = TextComponent(
  text: '+1 kr',
  textRenderer: TextPaint(style: const TextStyle(fontSize: 24)),
)
  ..add(MoveByEffect(Vector2(0, -60), EffectController(duration: 0.8)))
  ..add(OpacityEffect.fadeOut(EffectController(duration: 0.5, startDelay: 0.3)));

The text rises smoothly for 0.3 seconds, then jumps up by roughly the ascent when the fade begins.

Expected behavior

Paint changes only affect paint; the glyphs stay where they are.

Suggested fix

Have onChanged apply the same ascent translation as updateBounds (or call a shared helper), for example:

@override
void onChanged() {
  _textRenderer = _textRenderer.copyWithPaint(paint) as T;
  _updateElement();
  _textElement.translate(0, _textElement.metrics.ascent);
}

Observed on the perf/component-set-backing branch; the implementation is the same on main.