task-18: 统一图片 spool 生命周期，超时/取消/异常路径删除临时文件

- ImageSpool 增加 closed 状态：close 幂等、close 后 put 拒绝且消息可识别，
  杜绝关闭后再写入导致目录删除失败
- 长预算 prefetchToDisk 任务体补 put 前中断检查：deadline/中断取消后不再
  产生新临时文件，close 可完整删除目录（与 best-effort 路径对齐）
- 长预算路径并入测试 hook 探测，与短预算路径行为一致，生命周期可测

backend-java/src/main/java/com/nanri/aiimage/modules/similarasin/util/SimilarAsinImageEmbedder.java

@@ -485,11 +485,15 @@ public class SimilarAsinImageEmbedder {
             try {
                 ResizedImage thumb = readLocalCachedThumb(url);
                 if (thumb == null) {
-                    thumb = fetchAndResizeDirect(url);
-                    downloaded.incrementAndGet();
+                    thumb = fetchAndResizeDirectForTest(url);
+                    if (thumb == null) {
+                        thumb = fetchAndResizeDirect(url);
+                        downloaded.incrementAndGet();
+                    }
                 } else {
                     localHit.incrementAndGet();
                 }
+                ensureImageWorkNotInterrupted();
                 imageSpool.put(url, thumb);
             } catch (Exception ex) {
                 failed.incrementAndGet();
@@ -1233,6 +1237,7 @@ public class SimilarAsinImageEmbedder {
 
         private final Path directory;
         private final ConcurrentMap<String, SpoolImage> images = new ConcurrentHashMap<>();
+        private volatile boolean closed;
 
         public ImageSpool(Path directory) throws IOException {
             this.directory = Objects.requireNonNull(directory, "directory must not be null")
@@ -1240,6 +1245,11 @@ public class SimilarAsinImageEmbedder {
             Files.createDirectories(this.directory);
         }
 
+        /** Task 18：暴露临时目录，供生命周期校验与日志。 */
+        public Path directory() {
+            return directory;
+        }
+
         public SpoolImage get(String url) {
             if (url == null) {
                 return null;
@@ -1249,6 +1259,9 @@ public class SimilarAsinImageEmbedder {
         }
 
         public SpoolImage put(String url, ResizedImage image) throws IOException {
+            if (closed) {
+                throw new IOException("image spool closed: " + directory);
+            }
             if (url == null || url.isBlank() || image == null || image.bytes() == null || image.bytes().length == 0) {
                 throw new IOException("invalid image spool entry");
             }
@@ -1281,6 +1294,7 @@ public class SimilarAsinImageEmbedder {
 
         @Override
         public void close() throws IOException {
+            closed = true;
             images.clear();
             if (!Files.exists(directory)) {
                 return;