fix(tools): validate concurrency params to prevent zero-semaphore deadlock

Codex 最终审查发现 video_concurrency=0 或 TREE_BUILD_API_CONCURRENCY=0
会创建永不放行的 Semaphore 导致整批挂死。追加 >= 1 校验，fail-fast 报错。

tools/build_trees.py

@@ -265,6 +265,10 @@ async def main_async(args: argparse.Namespace) -> None:
     assert videos_dir.is_dir(), f"视频目录不存在: {videos_dir}"
 
     api_concurrency = int(os.getenv("TREE_BUILD_API_CONCURRENCY", "16"))
+    if api_concurrency < 1:
+        raise ValueError(f"TREE_BUILD_API_CONCURRENCY 必须 >= 1，当前值: {api_concurrency}")
+    if args.video_concurrency < 1:
+        raise ValueError(f"--video-concurrency 必须 >= 1，当前值: {args.video_concurrency}")
 
     # Phase 1: 待建发现（progress + 完整性双重跳过）
     progress_path = Path(args.progress_path)