#2376 setup: give the --families probe the engine's ROCm DLLs on PATH (#2372)

本文

Closes #2372.

A ready-made Windows HIP engine keeps `hipblas.dll` and `libhipblaslt.dll` in `rocm/bin` beside it, and its loader asks for them **before `main`**. `engine_families()` launched `strata.exe --families` with no environment, so on Windows both DLLs were reported missing, `--families` printed nothing, and setup read that as "this engine does not run Qwen3.6-35B-A3B yet" — while the engine was already current. The report's diagnosis, and the shape of the fix.

`hip_devices()` already builds that PATH for the device probe (`hip_lib_dirs(eng)` prepended to the inherited `PATH`); the families probe now gets the same one.

## Testing

New `tools/test_setup_engine_families.py` — hermetic (a temporary engine directory with a `BUILD.json`, `subprocess.run` captured; no engine, no GPU, no network):

- the probe is launched with `env["PATH"]` starting at the engine's `rocm/bin`, and the inherited environment kept;
- the command is still `<engine>/<EXE> --families`;
- an engine whose `BUILD.json` lists no `lib_dirs` keeps the inherited `PATH` unchanged;
- empty output still yields the Flash-Next default (`{"qwen4exp"}`).

Removing the `env=` argument fails two of the three (`failures=1, errors=1`) — `the probe ran without an environment` — so the guard is the reported bug rather than a restatement.

The rest of `tools/test_setup_*.py` is unchanged (37 pass; `test_setup_engine_hash` and `test_setup_golden` fail identically without this change).