feat(devicemanager): add selector-based device APIs, selectFeature helper, convenience discovery methods, and ESCL scan completion fallback

ts/protocols/protocol.escl.ts

@@ -142,13 +142,28 @@ export class EsclProtocol {
 
   /**
    * Wait for scan job to complete and download the result
+   *
+   * Note: Many scanners (including Brother) don't start scanning until
+   * NextDocument is requested. The request triggers the scan and blocks
+   * until complete. We try direct download first, then fall back to polling.
    */
   public async waitForScanComplete(
     jobUri: string,
     options: IScanOptions,
     pollInterval: number = 500
   ): Promise<IScanResult> {
-    // Poll until job is complete
+    // Try direct download first - this triggers the scan on many scanners
+    // (including Brother) and blocks until the scan completes
+    try {
+      const result = await this.downloadScan(jobUri, options);
+      if (result.data.length > 0) {
+        return result;
+      }
+    } catch (err) {
+      // Direct download failed, fall back to polling
+    }
+
+    // Fall back to polling for scanners that need it
     let attempts = 0;
     const maxAttempts = 120; // 60 seconds max