feat(canvas): move image pipeline rendering off main thread with worker fallback

lib/image-pipeline/source-loader.ts

@@ -1,6 +1,19 @@
 const imageBitmapCache = new Map<string, Promise<ImageBitmap>>();
 
-export async function loadSourceBitmap(sourceUrl: string): Promise<ImageBitmap> {
+type LoadSourceBitmapOptions = {
+  signal?: AbortSignal;
+};
+
+function throwIfAborted(signal: AbortSignal | undefined): void {
+  if (signal?.aborted) {
+    throw new DOMException("The operation was aborted.", "AbortError");
+  }
+}
+
+export async function loadSourceBitmap(
+  sourceUrl: string,
+  options: LoadSourceBitmapOptions = {},
+): Promise<ImageBitmap> {
   if (!sourceUrl || sourceUrl.trim().length === 0) {
     throw new Error("Render sourceUrl is required.");
   }
@@ -9,6 +22,19 @@ export async function loadSourceBitmap(sourceUrl: string): Promise<ImageBitmap>
     throw new Error("ImageBitmap is not available in this environment.");
   }
 
+  throwIfAborted(options.signal);
+
+  if (options.signal) {
+    const response = await fetch(sourceUrl, { signal: options.signal });
+    if (!response.ok) {
+      throw new Error(`Render source failed: ${response.status}`);
+    }
+
+    const blob = await response.blob();
+    throwIfAborted(options.signal);
+    return await createImageBitmap(blob);
+  }
+
   const cached = imageBitmapCache.get(sourceUrl);
   if (cached) {
     return await cached;