3D 冒烟示例把产物写到 target 目录，不再落到仓库里

- server-rs/crates/platform-tripo/examples/tripo_generation_smoke.rs：产物落盘目录从「当前工作目录」改为 <cargo target>/tripo-smoke-artifacts，开始前建目录并打印路径；该目录本来就在入库排除范围内，根目录不会再攒出临时模型
- server-rs/crates/platform-tripo/examples/tripo_generation_smoke.rs：文件头文档同步改为说明新的产物落点

server-rs/crates/platform-tripo/examples/tripo_generation_smoke.rs

@@ -9,10 +9,12 @@
 //! ```
 //!
 //! The example waits for each submitted task to finish, prints its mapped
-//! output, and writes the downloaded model into the current directory. The
-//! SDK poller is used only here and is not exposed by `platform-tripo`.
+//! output, and writes the downloaded model into
+//! `<cargo target dir>/tripo-smoke-artifacts` so a smoke run never leaves
+//! tens-of-MB models inside the repository tree. The SDK poller is used only
+//! here and is not exposed by `platform-tripo`.
 
-use std::{env, time::Duration};
+use std::{env, path::PathBuf, time::Duration};
 
 use platform_tripo::{TripoImageInput, TripoProviderClient};
 use shared_contracts::model3d::{
@@ -31,6 +33,14 @@ const SAMPLE_IMAGE_URL: &str = "https://www.rustacean.net/assets/rustacean-flat-
 /// 冒烟脚本单次允许的产物体积上限，与 api-server 的口径一致（512 MiB）。
 const SMOKE_MAX_ARTIFACT_BYTES: u64 = 512 * 1024 * 1024;
 
+/// 产物落盘目录：固定在 cargo 的 target 目录下（`server-rs/target`），
+/// 那里本来就不入库，示例也就不会在仓库里留下几十 MB 的临时模型。
+fn artifact_output_dir() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("../../target")
+        .join("tripo-smoke-artifacts")
+}
+
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let api_key = required_env("TRIPO_API_KEY")?;
@@ -140,6 +150,10 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .await?;
     println!("multiview_to_model task_id={}", multiview.task_id);
 
+    let output_dir = artifact_output_dir();
+    tokio::fs::create_dir_all(&output_dir).await?;
+    println!("artifact output dir={}", output_dir.display());
+
     for (name, handle) in [
         ("text_to_model", text),
         ("image_to_model", image),
@@ -198,11 +212,12 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             }
         };
         let filename = artifact.filename(name);
-        let mut file = File::create(&filename).await?;
+        let path = output_dir.join(&filename);
+        let mut file = File::create(&path).await?;
         file.write_all(&artifact.bytes).await?;
         println!(
             "{name} artifact={} bytes={} url={}",
-            filename,
+            path.display(),
             artifact.bytes.len(),
             artifact.url.redacted()
         );