FadeBox class final

The render-safe fade primitive: one named place where every Fluvie fade becomes pixels.

Paint-equivalent to a raw Opacity by construction:

• at exactly 1.0 it mounts nothing — the child paints directly, exactly as Opacity at 1 would paint it (and as the keyframe applier paints an element with no opacity at all);
• everywhere else it mounts a single Opacity with opacity clamped into [0, 1] (spring overshoot can push a composed keyframe past the bounds mid-flight);
• at 0 the Opacity holds the layout slot while RenderOpacity skips painting entirely — hidden elements never collapse the layout.

The keyframe applier and MotionTarget's collect-pass placeholder build this instead of Opacity directly, so future fade upgrades (such as a shader fade) land in exactly one widget.