fix(tools): widen error isolation to cover full single-video pipeline

Codex 功能质量审查发现 _build_one 的 try/except 仅包裹 build_async，
构造器、SRT 解析、save_json、save_progress 抛异常会逃逸到 gather 中止整批。
将 try 边界扩大到覆盖完整单视频流程（构建→建树→保存→进度记录）。

tools/build_trees.py

@@ -300,21 +300,21 @@ async def main_async(args: argparse.Namespace) -> None:
         async with video_sem:
             vid = video_path.stem
             logger.info("开始建树 {}", vid)
+            try:
                 builder = VideoTreeBuilder(vlm=vlm, llm=llm, config=tree_cfg, api_semaphore=api_sem)
                 srt_entries = _find_srt_entries(video_path, srt_dir)
-            try:
                 index = await builder.build_async(str(video_path), srt_entries)
+                tree_path = out_dir / vid / "tree.json"
+                tree_path.parent.mkdir(parents=True, exist_ok=True)
+                index.save_json(str(tree_path))
+                await save_progress(progress_path, progress_lock, vid)
+                completed += 1
             except Exception as exc:
                 # WHY: 单视频错误隔离——一个视频失败不拖垮整批；
                 # 失败清单汇总上报，vid 不进 progress，下次重跑自动重建。
                 logger.error("建树失败 {} ({}): {}", vid, type(exc).__name__, exc)
                 failed.append(vid)
                 return
-            tree_path = out_dir / vid / "tree.json"
-            tree_path.parent.mkdir(parents=True, exist_ok=True)
-            index.save_json(str(tree_path))
-            await save_progress(progress_path, progress_lock, vid)
-            completed += 1
             if completed % 5 == 0:
                 elapsed = time.time() - start_time
                 rate = completed / elapsed * 60 if elapsed > 0 else 0