refactor(scene): paint decorative fullscreen through the box tree

[LeadcodeDev]

Severity Low, category correctness. Location: crates/rustmotion/src/engine/render/scene.rs:595

Impact

Every non-decorative node is timed by PaintWindow::contains — self.end.is_none_or(|e| t < e) (rustmotion-core/src/engine/box_tree.rs:43), a half-open window — and gets its effects from box_builder::effective_effects (legacy_dispatch.rs:110), which folds in timeline steps and style.transition keyframes. This second path uses an inclusive time > e and calls a.animation_effects() raw. Particle declares timeline: Vec<TimelineStep> (particle.rs:57), so a particle's timeline steps and transition keyframes are silently dropped, and its end_at keeps it visible for one extra frame — but only in a world view, since this is the sole call site (scene.rs:1130). The same particle in a slide view behaves correctly. A component whose behaviour depends on which view type contains it is invisible to tests written against either one.

Fix

Give decorative children a real BoxNode with position: absolute; width/height: 100% and let them flow through paint_tree like everything else, deleting paint_decorative_fullscreen. If the bypass must stay, have it call effective_effects and PaintWindow::contains rather than re-deriving both.

Evidence the audit read

if let Some(e) = end_at {
            if time > e { return; }
        }
    }
    let props = match child.component.as_animatable() {
        Some(a) => {
            let effects = a.animation_effects();
            if effects.is_empty() { AnimatedProperties::default() }
            else { resolve_props_for_effects(effects, time, ctx.scene_duration) }

Stacked on fix/heropattern-scroll-period, which carries the previous finding of this workstream. GitHub shows only this finding's diff; merge in order.

Part of the September 2026 audit remediation chantier. Refs #220 (RM-36).

[LeadcodeDev]

Superseded by #283. The workstream's finding order was declared wrong in the carving table, so this pull request was opened on the wrong base. #283 carries the same finding on the correct one.