Phase 4: prefetch upcoming images during playback

`assets_needed_at(document, time)` (core) enumerates the image asset ids referenced by
the visible vector layers' active keyframes at a time (top-level + group children).
During playback the stage decodes the images needed ~0.5s ahead into the bounded
ImageCache, so a keyframe that swaps image fills doesn't hitch when the playhead
reaches it. Gated on is_playing; nested clip-instance recursion + background decode
are refinements.

Completes Phase 4 (image asset paging: Tier 2 decoded-cache LRU, Tier 1 lazy bytes,
playback prefetch).

lightningbeam-ui/lightningbeam-core/src/renderer.rs

@@ -158,6 +158,28 @@ impl Default for ImageCache {
     }
 }
 
+/// Image asset ids referenced by the visible vector layers' active keyframes at `time`
+/// (top-level + group children). Used to prefetch/decode images ahead during playback.
+/// (Recursing into nested clip instances is a refinement.)
+pub fn assets_needed_at(document: &Document, time: f64) -> Vec<Uuid> {
+    let mut ids = Vec::new();
+    for layer in document.all_layers() {
+        if let crate::layer::AnyLayer::Vector(vl) = layer {
+            if !vl.layer.visible {
+                continue;
+            }
+            if let Some(kf) = vl.keyframe_at(time) {
+                for fill in &kf.graph.fills {
+                    if let Some(id) = fill.image_fill {
+                        ids.push(id);
+                    }
+                }
+            }
+        }
+    }
+    ids
+}
+
 /// Decode image bytes to a premultiplied tiny-skia Pixmap (CPU render path).
 fn decode_image_to_pixmap(data: &[u8]) -> Option<tiny_skia::Pixmap> {
     let img = image::load_from_memory(data).ok()?;

lightningbeam-ui/lightningbeam-editor/src/panes/stage.rs

@@ -439,6 +439,8 @@ struct VelloRenderContext {
     onion: crate::panes::OnionSkinSettings,
     /// `.beam` container path for lazily paging image-asset bytes in the ImageCache.
     container_path: Option<std::path::PathBuf>,
+    /// Whether playback is active (gates image prefetch).
+    is_playing: bool,
     /// Active layer for tool operations
     active_layer_id: Option<uuid::Uuid>,
     /// Delta for drag preview (world space)
@@ -1047,6 +1049,22 @@ impl egui_wgpu::CallbackTrait for VelloCallback {
                 }
             }
 
+            // Prefetch: during playback, decode images the upcoming frames will need
+            // (a short lookahead) into the bounded cache, so a keyframe that swaps image
+            // fills doesn't hitch when the playhead reaches it.
+            if self.ctx.is_playing {
+                const PREFETCH_LOOKAHEAD: f64 = 0.5;
+                let ahead = lightningbeam_core::renderer::assets_needed_at(
+                    &self.ctx.document,
+                    self.ctx.playback_time + PREFETCH_LOOKAHEAD,
+                );
+                for id in ahead {
+                    if let Some(asset) = self.ctx.document.get_image_asset(&id) {
+                        let _ = image_cache.get_or_decode(asset);
+                    }
+                }
+            }
+
             drop(image_cache);
             let _t_after_scene_build = std::time::Instant::now();
 
@@ -12078,6 +12096,7 @@ impl PaneRenderer for StagePane {
             tool_state: shared.tool_state.clone(),
             onion: shared.onion,
             container_path: shared.container_path.clone(),
+            is_playing: *shared.is_playing,
             active_layer_id: *shared.active_layer_id,
             drag_delta,
             selection: shared.selection.clone(),