Harden ROCm detection, Radeon wheel fallback, and HIP visibility

Addresses review findings from parallel reviewers on PR #4720:

- install.sh: add _has_usable_nvidia_gpu() helper requiring nvidia-smi -L
  to actually list a GPU before treating the host as NVIDIA. Fixes the
  stale-nvidia-smi-on-PATH regression where AMD-only hosts fell into the
  CUDA branch.
- install.sh: fix hipconfig awk blocks to propagate a non-zero exit code
  when the output is not a recognisable version string, so the ||-chain
  continues to dpkg-query / rpm instead of terminating early.
- install.sh: fail-closed on Radeon wheel fallback. When torch,
  torchvision or torchaudio is missing from the Radeon repo for the
  active Python tag, fall back to the standard ROCm index instead of
  silently mixing Radeon wheels with PyPI defaults. Quote all wheel
  arguments individually so wheel filenames cannot be word-split or
  glob-expanded.
- install_llama_prebuilt.py: detect_host() now requires nvidia-smi -L to
  list a GPU before setting has_physical_nvidia. Routes AMD ROCm hosts
  with a broken leftover nvidia-smi to the ROCm path instead of
  misclassifying them as NVIDIA.
- install_llama_prebuilt.py: scan upstream assets for any rocm-<version>
  prebuilt instead of hard-coding rocm-7.2, so ROCm 6.x / 7.0 / 7.1 / 7.3+
  users pick up a matching upstream prebuilt when one exists.
- install_llama_prebuilt.py: validate_server() adds --n-gpu-layers 1 for
  linux-rocm and windows-hip hosts, so new HIP prebuilts are preflighted
  on the GPU path instead of passing validation on CPU only.
- install_llama_prebuilt.py: restore the published windows-cpu fallback
  for AMD Windows hosts without a HIP prebuilt so hash-approved bundles
  are still preferred over the raw upstream CPU asset.
- install_python_stack.py: drop the /opt/rocm / hipcc gate in
  _ensure_rocm_torch() and rely on _has_rocm_gpu(). Runtime-only ROCm
  installs (package-managed minimal installs, Radeon software) that ship
  amd-smi / rocminfo without hipcc can now repair a CPU-only venv via
  "unsloth studio update". Adds an explicit IS_WINDOWS / IS_MACOS guard.
- studio/backend/utils/hardware/amd.py: honour HIP_VISIBLE_DEVICES /
  ROCR_VISIBLE_DEVICES / CUDA_VISIBLE_DEVICES in
  get_primary_gpu_utilization(). A process restricted to GPU 2 now
  reports metrics for GPU 2 instead of physical GPU 0. Tighten the plain
  bytes unit detection to an explicit allowlist.
- studio/backend/utils/hardware/hardware.py: route
  get_backend_visible_gpu_info()'s backend_cuda_visible_devices field
  through a helper that reads HIP_VISIBLE_DEVICES on ROCm. Drop the
  unconditional "(rocm=False)" suffix in apply_gpu_ids() logs.

studio/install_python_stack.py

@@ -157,20 +157,28 @@ def _has_usable_nvidia_gpu() -> bool:
 def _ensure_rocm_torch() -> None:
     """Reinstall torch with ROCm wheels when the venv received CPU-only torch.
 
-    Runs only on Linux hosts where ROCm is installed and an AMD GPU is
-    present.  No-op when torch already links against HIP (ROCm) or CUDA
-    (NVIDIA).  Skips on Windows/macOS and on mixed AMD+NVIDIA hosts
-    (NVIDIA takes precedence).
+    Runs only on Linux hosts where an AMD GPU is present and the ROCm
+    runtime is detectable (rocminfo / amd-smi / hipconfig / rocm-core
+    package).  No-op when torch already links against HIP (ROCm) or on
+    Windows/macOS or on mixed AMD+NVIDIA hosts (NVIDIA takes precedence).
     Uses pip_install() to respect uv, constraints, and --python targeting.
     """
+    # Explicit OS guard so the helper is safe to call from any context --
+    # ROCm wheels are only published for Linux x86_64.
+    if IS_WINDOWS or IS_MACOS:
+        return
     # NVIDIA takes precedence on mixed hosts -- but only if an actual GPU is usable
     if _has_usable_nvidia_gpu():
         return
-    rocm_root = os.environ.get("ROCM_PATH") or "/opt/rocm"
-    if not os.path.isdir(rocm_root) and not shutil.which("hipcc"):
-        return  # no ROCm toolchain
+    # Rely on _has_rocm_gpu() (rocminfo / amd-smi GPU data rows) as the
+    # authoritative "is this actually an AMD ROCm host?" signal. The old
+    # gate required /opt/rocm or hipcc to exist, which breaks on
+    # runtime-only ROCm installs (package-managed minimal installs,
+    # Radeon software) that ship amd-smi/rocminfo without /opt/rocm or
+    # hipcc, and leaves `unsloth studio update` unable to repair a
+    # CPU-only venv on those systems.
     if not _has_rocm_gpu():
-        return  # ROCm tools present but no AMD GPU
+        return  # no AMD GPU visible
 
     ver = _detect_rocm_version()
     if ver is None: