bench: report when a foreign vendor's FlagGems implementation gets loaded

FlagGems keeps per-vendor implementations under
flag_gems/runtime/backend/_<vendor>/ops/, and benchmarks commonly gate on
whether one imports:

    try:
        from flag_gems.runtime.backend._nvidia.hopper.ops.mm import router_gemm
        ROUTER_GEMM_AVAILABLE = True
    except Exception:
        ROUTER_GEMM_AVAILABLE = False

An import only proves the .py is on disk. It touches no device, so it succeeds
on every backend and the gate never fires. test_router_gemm.py therefore ran
the NVIDIA Hopper kernel on an S5000: its autotune and heuristics are keyed to
nvidia, supply no BLOCK_M/N/K/SPLIT_K on musa, and the run died deep inside the
JIT with "dynamic_func() missing 4 required positional arguments" — an error
that points nowhere near the cause. Meanwhile _mthreads/ops/mm.py holds a
working router_gemm the benchmark never imports. Gating on vendor_name first,
the way the Cambricon branch in the same file already does, makes it run: 10
shapes, 10 kernels, cvt 10 -> 2.

The plugin now scans sys.modules at session end and prints
FOREIGN_VENDOR_MODULE with the vendors it found. It reports rather than
refuses: loading another vendor's module is not proof of a problem, but when a
result looks wrong it is the first line to check. Vendor roots are compared
segment-wise, not by prefix, so a vendor whose name extends ours is not
mistaken for our own.

README gains the ordered checklist for an operator that will not run, with the
real instance behind each entry.

_device_setup_plugin.py

@@ -67,3 +67,46 @@ def pytest_configure(config):
 
 
 _setup_at_import()
+
+# FlagGems 的厂商实现按 flag_gems.runtime.backend._<vendor>.ops.* 组织。benchmark
+# 常写成 `try: import <某厂商实现>; AVAILABLE = True`——但 import 只证明那个 .py 在
+# 磁盘上，它不碰设备，所以在任何后端上都会成功。于是 gate 形同虚设，测出来的是别家
+# 的 kernel 跑在本机上。router_gemm 就是这样：test_router_gemm.py 无条件导入
+# _nvidia.hopper 版，在 MUSA 上 gate 照过，最后死在 JIT 深处的
+# "dynamic_func() missing BLOCK_M, BLOCK_N, BLOCK_K, SPLIT_K"——那个报错完全指不
+# 向真正的原因，而 _mthreads 下明明有一份能用的实现。
+#
+# 这里不拦，只报：跑完看一眼载入过谁家的实现。
+_VENDOR_MOD_PREFIX = "flag_gems.runtime.backend._"
+
+
+def foreign_vendor_modules(active_vendor: str, modules=None) -> list:
+    """本次进程里载入过的、不属于当前厂商的 FlagGems 后端模块。"""
+    if modules is None:
+        modules = sys.modules
+    out = set()
+    for name in list(modules):
+        if not name.startswith(_VENDOR_MOD_PREFIX):
+            continue
+        # 只保留厂商根，_nvidia.hopper.ops.mm 与 _nvidia.ops.mm 算同一条。
+        # 按段比而不是按前缀：前缀会让 _<vendor>_something 被误认成自家。
+        root = name[len(_VENDOR_MOD_PREFIX):].split(".")[0]
+        if root != active_vendor:
+            out.add(root)
+    return sorted(out)
+
+
+def pytest_sessionfinish(session, exitstatus):
+    from _term_style import tag
+    try:
+        vendor = get_backend().flaggems_vendor
+    except Exception:
+        return
+    foreign = foreign_vendor_modules(vendor)
+    if not foreign:
+        return
+    print(f"{tag('[device-setup-plugin]')} FOREIGN_VENDOR_MODULE active={vendor} "
+          f"loaded={','.join(foreign)}", file=sys.stderr, flush=True)
+    print(f"{tag('[device-setup-plugin]')} 本次载入了别家厂商的 FlagGems 实现。"
+          "如果这个算子的结果可疑，先查 benchmark 的可用性判据是不是只做了 import "
+          "而没判 vendor_name", file=sys.stderr, flush=True)