Keyframe diamonds: pointing-hand cursor + prefetch during playback

- Pointing-hand cursor when hovering a clickable keyframe diamond.
- Prefetch (Phase 3e, playback only): each update during playback, page in the next
  few upcoming keyframes (PREFETCH_AHEAD=4) per raster layer that aren't resident, via
  the existing async worker. Their full pixels land before the playhead reaches them,
  so playback shows full frames instead of the low-res proxy on every frame (the
  proxy→full pop was the "flicker"). Reactive faults still cover scrubbing.

lightningbeam-ui/lightningbeam-editor/src/main.rs

@@ -5283,13 +5283,30 @@ impl eframe::App for EditorApp {
                 }
             }
 
-            // 2) Dispatch background page-ins for newly-requested keyframes.
+            // 2) Dispatch background page-ins: reactive misses, plus (during playback)
-            let wanted: Vec<uuid::Uuid> = {
+            //    a prefetch of upcoming keyframes so their full pixels are resident
+            //    before the playhead reaches them — otherwise every frame shows the
+            //    low-res proxy and the proxy→full pops read as flicker.
+            let mut to_load: Vec<uuid::Uuid> = {
                 let mut s = self.raster_fault_requests.lock().unwrap();
                 s.drain().collect()
             };
+            if self.is_playing && self.raster_store.has_path() {
+                const PREFETCH_AHEAD: usize = 4;
+                let t = self.playback_time;
+                let doc = self.action_executor.document();
+                for layer in doc.all_layers() {
+                    if let lightningbeam_core::layer::AnyLayer::Raster(rl) = layer {
+                        for kf in rl.keyframes.iter().filter(|kf| kf.time >= t).take(PREFETCH_AHEAD) {
+                            if kf.raw_pixels.is_empty() && kf.needs_fault_in {
+                                to_load.push(kf.id);
+                            }
+                        }
+                    }
+                }
+            }
             if self.raster_store.has_path() {
-                for kf_id in wanted {
+                for kf_id in to_load {
                     if self.raster_loads_inflight.contains(&kf_id) {
                         continue;
                     }

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

@@ -4867,6 +4867,13 @@ impl TimelinePane {
             }
         }
 
+        // Pointing-hand cursor when hovering a clickable keyframe diamond.
+        if let Some(hover) = response.hover_pos() {
+            if self.keyframe_diamond_hits.iter().any(|(r, _)| r.contains(hover)) {
+                ui.ctx().set_cursor_icon(egui::CursorIcon::PointingHand);
+            }
+        }
+
         // Click a keyframe diamond → snap the playhead exactly to that keyframe.
         // (Hit targets come from the previous frame's render_layers; diamonds don't
         // move between frames, so a click lands on the right one.)