Lock in a stable official route for real FMA archive acquisition

Constraint: Real-data progress was blocked until we could prove an upstream archive path that still works today
Rejected: Continue iterating on historical per-track URLs | Those paths already proved unstable via 403 and 404 evidence
Confidence: high
Scope-risk: narrow
Directive: Prefer the verified fma_small.zip archive route over legacy page or single-track scraping paths unless upstream changes again
Tested: curl -I -L --max-time 60 https://os.unil.cloud.switch.ch/fma/fma_small.zip; curl -L --range 0-1023 --max-time 60 -o /tmp/fma_small_probe.bin https://os.unil.cloud.switch.ch/fma/fma_small.zip
Not-tested: Full 7.68 GB archive download, extraction, and smoke execution remain pending

@@ -225,6 +225,27 @@
 
 
 
+
+### Stage: FMA 官方整包下载路径确认
+
+完成项：
+- 进一步验证 FMA Small 的稳定 archive 下载路径
+- 确认 `fma_small.zip` 可通过官方归档地址直接访问
+- 将该路径补充到开放数据工作流与交接文档
+
+验证结果：
+- `curl -I -L --max-time 60 https://os.unil.cloud.switch.ch/fma/fma_small.zip` 成功
+- 当前响应头关键信息：
+  - `200 OK`
+  - `Content-Type: application/zip`
+  - `Content-Length: 7679594875`
+  - `Accept-Ranges: bytes`
+- `curl -L --range 0-1023 ...` 成功获取前 `1024` bytes
+
+结论：
+- FMA 真实数据已经不再缺下载地址
+- 当前剩余问题从“找不到稳定 URL”转为“是否开始实际拉取 7.68 GB 归档并落盘”
+
 ### Stage: FMA 下载器模块调用修复
 
 完成项：

@@ -276,7 +276,7 @@
    - [docs/session-handoff.md](./session-handoff.md)
    - [docs/current-capability-map.md](./current-capability-map.md)
    - [acr-engine/FIRST_RUN_CHECKLIST.md](../acr-engine/FIRST_RUN_CHECKLIST.md)
-- FMA 真实子集下载脚手架已存在：[acr-engine/scripts/fetch_fma_subset.py](../acr-engine/scripts/fetch_fma_subset.py)；最近验证结果是旧直链 `403`、随后已修复为 `python -m yt_dlp` 调用，但页面级历史 URL 又返回 `404`
+- FMA 真实子集下载脚手架已存在：[acr-engine/scripts/fetch_fma_subset.py](../acr-engine/scripts/fetch_fma_subset.py)；最近验证结果是旧直链 `403`、页面级历史 URL `404`；但 `https://os.unil.cloud.switch.ch/fma/fma_small.zip` 已验证 `200 OK` 且支持 range
    - 运行 [acr-engine/scripts/status_snapshot.py](../acr-engine/scripts/status_snapshot.py)
    - 或直接查看最新落盘快照：`acr-engine/.omx/latest_status_snapshot.json`