Phase 3a-2: async raster page-in (no UI block)

The fault-in drain no longer decodes on the UI thread. It now:
- dispatches each newly-requested keyframe's page-in to a background thread
  (deduped via an in-flight set, store cloned per request so path changes are
  picked up), and
- applies completed results from a channel at the top of update(), keeping the UI
  ticking while loads are outstanding.

Cold scrubs no longer freeze. The brief blank gap before a frame lands is removed
by the image proxy (3a-3); eviction to bound RAM while scrubbing is 3b.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -1032,6 +1032,13 @@ struct EditorApp {
     /// resident. Drained and faulted in at the top of the next `update()`.
     raster_fault_requests:
         std::sync::Arc<std::sync::Mutex<std::collections::HashSet<uuid::Uuid>>>,
+    /// Async raster page-in: background decode results `(kf_id, pixels-or-None)`.
+    /// `None` = the keyframe genuinely isn't in the container (blank).
+    raster_load_result_tx: std::sync::mpsc::Sender<(uuid::Uuid, Option<Vec<u8>>)>,
+    raster_load_result_rx: std::sync::mpsc::Receiver<(uuid::Uuid, Option<Vec<u8>>)>,
+    /// Keyframe ids whose page-in is currently running on a background thread
+    /// (prevents duplicate loads while the canvas keeps re-requesting them).
+    raster_loads_inflight: std::collections::HashSet<uuid::Uuid>,
     /// Application configuration (recent files, etc.)
     config: AppConfig,
     /// Remappable keyboard shortcut manager
@@ -1201,6 +1208,7 @@ impl EditorApp {
             .unwrap_or(wgpu::TextureFormat::Rgba8Unorm);
 
         let (waveform_result_tx, waveform_result_rx) = std::sync::mpsc::channel();
+        let (raster_load_result_tx, raster_load_result_rx) = std::sync::mpsc::channel();
 
         Self {
             layouts,
@@ -1297,6 +1305,9 @@ impl EditorApp {
             current_file_path: None, // No file loaded initially
             raster_store: lightningbeam_core::raster_store::RasterStore::new(None),
             raster_fault_requests: Default::default(),
+            raster_load_result_tx,
+            raster_load_result_rx,
+            raster_loads_inflight: Default::default(),
             keymap: KeymapManager::new(&config.keybindings),
             config,
             file_command_tx,
@@ -5149,15 +5160,11 @@ impl eframe::App for EditorApp {
         // any rendering, and fault the pixels in from the project container. A drain of
         // an empty set is ~free, so this is cheap when everything is already resident.
         {
-            let wanted: Vec<uuid::Uuid> = {
-                let mut s = self.raster_fault_requests.lock().unwrap();
-                s.drain().collect()
-            };
-            if !wanted.is_empty() && self.raster_store.has_path() {
-                let mut any_loaded = false;
+            // 1) Apply completed background page-ins.
+            let mut any_applied = false;
+            while let Ok((kf_id, result)) = self.raster_load_result_rx.try_recv() {
+                self.raster_loads_inflight.remove(&kf_id);
                 let doc = self.action_executor.document_mut();
-                for kf_id in wanted {
-                    // Find the keyframe by id across every raster layer in the document.
                 let kf = doc.all_layers_mut().into_iter().find_map(|l| match l {
                     lightningbeam_core::layer::AnyLayer::Raster(rl) => {
                         rl.keyframes.iter_mut().find(|kf| kf.id == kf_id)
@@ -5165,21 +5172,46 @@ impl eframe::App for EditorApp {
                     _ => None,
                 });
                 if let Some(kf) = kf {
-                        if kf.raw_pixels.is_empty() && kf.needs_fault_in {
-                            if let Some(pixels) = self.raster_store.load_pixels(kf_id) {
+                    // Don't clobber pixels edited while the load was in flight.
+                    if kf.needs_fault_in && kf.raw_pixels.is_empty() {
+                        if let Some(pixels) = result {
                             kf.raw_pixels = pixels;
                             kf.texture_dirty = true;
-                                any_loaded = true;
+                            any_applied = true;
                         }
-                            // Resolved (loaded, or genuinely absent/blank) — clear the
-                            // flag so we don't re-request a page-in every frame.
+                        // Resolved (loaded, or genuinely absent/blank).
                         kf.needs_fault_in = false;
                     }
                 }
             }
-                if any_loaded {
+            if any_applied {
                 ctx.request_repaint();
             }
+
+            // 2) Dispatch background page-ins for newly-requested keyframes.
+            let wanted: Vec<uuid::Uuid> = {
+                let mut s = self.raster_fault_requests.lock().unwrap();
+                s.drain().collect()
+            };
+            if self.raster_store.has_path() {
+                for kf_id in wanted {
+                    if self.raster_loads_inflight.contains(&kf_id) {
+                        continue;
+                    }
+                    self.raster_loads_inflight.insert(kf_id);
+                    let store = self.raster_store.clone();
+                    let tx = self.raster_load_result_tx.clone();
+                    std::thread::spawn(move || {
+                        let pixels = store.load_pixels(kf_id);
+                        let _ = tx.send((kf_id, pixels));
+                    });
+                }
+            }
+
+            // Keep ticking while page-ins are running so their results get applied
+            // promptly (a background thread can't wake egui on its own).
+            if !self.raster_loads_inflight.is_empty() {
+                ctx.request_repaint();
             }
         }