feat(viewport): Translator — the half of a viewport that was missing

[tannevaled]

Clipping existed; coordinate translation did not. A viewport is both, and the
gap had a real cost.

With no way to say "draw my child 250 pixels higher", Clipper's only
customer — the toolkit's ScrollView — moved the child's bounds instead,
drew, and put them back:

cb := s.Child.Bounds()
s.Child.SetBounds(Rect{X: r.X - s.OffsetX, Y: r.Y - s.OffsetY, …})
s.Child.Draw(p, theme)
s.Child.SetBounds(cb)          // restored

Geometry that changes for the duration of a paint is invisible to anything
reading it from outside. The accessibility bridges read bounds between frames, so
a screen reader was told a control sat a quarter of a window below where it was
painted — and the toolkit needed a childOffsetter interface purely to
compensate.

A translation shifts the paint, not the widget: a child lays out and reports
its bounds wherever it genuinely is, and the viewport decides where those pixels
land. Nothing has to be moved and restored.

if t, ok := p.(painter.Translator); ok {
    t.PushTranslate(-offsetX, -offsetY)
    defer t.PopTranslate()
}
child.Draw(p, theme)

The part worth reviewing

The offset is applied in the one write every primitive funnels through —
PutPixel for pixels, set/setFg for cells — not in each public method.
Primitives compose: FillRoundRect → FillRect → PutPixel. Shifting per
public method would apply the offset once per layer, moving a rounded rectangle
three times as far as a pixel. TestTranslationAppliedOncePerWrite pins that
down.

A clip pushed while translated moves with it, since a caller expresses it in the
same coordinates as everything else it draws.

Optional capability, exactly like Clipper: a back-end that cannot translate
does not implement it and the assertion is skipped. Both PixelPainter and
CellPainter do.

Deliberately not included

Nothing is moved onto it here. The toolkit's ScrollView follows in its own
change, so if any rendering regresses there is one commit to point at. The
existing clip, path and golden tests all pass unchanged — with no translation
pushed the offset is zero and every write is byte-identical.

Package coverage stays at 100%.

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