Add Windows-specific ROCm/HIP detection in detect_host()

The previous detect_host() ROCm check used rocminfo and amd-smi list
which are Linux-only tools. On Windows, has_rocm would always be False,
making the Windows HIP prebuilt path at line 1794 unreachable.

Now detect_host() uses platform-specific detection:
- Linux: rocminfo (check for gfx GPU names) or amd-smi list
- Windows: hipinfo.exe, amd-smi, or amdhip64.dll on PATH

This allows Windows AMD users to get the HIP prebuilt binary instead
of silently falling through to the CPU prebuilt.

studio/install_llama_prebuilt.py

@@ -1433,7 +1433,7 @@ def detect_host() -> HostInfo:
 
     # Detect AMD ROCm (HIP) -- require actual GPU, not just tools installed
     has_rocm = False
-    if not is_macos:
+    if is_linux:
         for _cmd, _marker in (
             (["rocminfo"], "gfx"),
             (["amd-smi", "list"], "gpu"),
@@ -1449,6 +1449,16 @@ def detect_host() -> HostInfo:
                 if _marker in _result.stdout.lower():
                     has_rocm = True
                     break
+    elif is_windows:
+        # Windows: check for HIP runtime DLL or hipinfo tool
+        if shutil.which("hipinfo") or shutil.which("amd-smi"):
+            has_rocm = True
+        elif any(
+            Path(d).joinpath("amdhip64.dll").exists()
+            for d in os.environ.get("PATH", "").split(os.pathsep)
+            if d
+        ):
+            has_rocm = True
 
     return HostInfo(
         system = system,

tests/studio/install/test_rocm_support.py

@@ -403,6 +403,13 @@ class TestHostInfoRocm:
         source = inspect.getsource(prebuilt_mod.detect_host)
         assert "ROCM_PATH" in source or "rocm" in source.lower()
 
+    def test_detect_host_windows_rocm_detection(self):
+        """detect_host() source should have Windows-specific HIP detection."""
+        import inspect
+
+        source = inspect.getsource(prebuilt_mod.detect_host)
+        assert "hipinfo" in source or "amdhip64" in source
+
 
 # =============================================================================
 # TEST: install_python_stack.py -- _detect_rocm_version