fix(studio/rocm): gate ROCm-only side-effects on active torch runtime

Address five edge cases flagged during PR review:

1. studio/backend/main.py: BNB_ROCM_VERSION was set whenever HIP_PATH or
   ROCM_PATH was present in the environment. A Windows CUDA user who once
   installed the HIP SDK and reverted to a CUDA torch wheel still has those
   env vars set, so bitsandbytes would try to load libbitsandbytes_rocm72.dll
   against a CUDA torch and crash. Now probe torch.version.hip inside the
   env-var guard (worker.py already does this).

2. studio/backend/main.py: os.add_dll_directory returned handles were
   discarded. Per CPython docs, the directory leaves the DLL search list when
   the handle is garbage collected. Retain handles in module-level
   _ROCM_DLL_HANDLES list so they survive process lifetime.

3. studio/install_python_stack.py: _install_bnb_windows_rocm() returned None
   regardless of pip_install_try outcome, and the caller flipped
   _rocm_windows_torch_installed to True unconditionally. On a failed BNB
   install the post-install "manual install may be required" warning was
   suppressed and the user was misled. Helper now returns bool; caller gates
   on it.

4. studio/install_python_stack.py: _detect_windows_gfx_arch returned the raw
   capture group, so mixed-case hipinfo output ("Gfx1151") missed the
   lowercase keys in _GFX_TO_AMD_INDEX_ARCH and silently fell back to CPU
   torch. Lowercase the token.

5. studio/install_python_stack.py: UNSLOTH_ROCM_TORCH_INSTALLED=1 early-
   return trusted the env var even when the venv was wiped between runs.
   Subprocess-probe torch importability first; fall through to the full
   install path if the probe fails.

Tests: 231 passed, 1 skipped in tests/studio/install/test_rocm_support.py
(adds one new test for case 5 fall-through).

studio/install_python_stack.py

@@ -259,7 +259,10 @@ def _detect_windows_gfx_arch() -> str | None:
             return None
         text = result.stdout.decode(errors = "replace")
         m = re.search(r"(?im)^\s*gcnArchName\s*:\s*(\S+)", text)
-        return m.group(1).strip() if m else None
+        # Lowercase the captured token -- some hipinfo builds emit "Gfx1151"
+        # which would miss the lowercase keys in _GFX_TO_AMD_INDEX_ARCH and
+        # silently fall back to CPU torch.
+        return m.group(1).strip().lower() if m else None
     except Exception:
         return None
 
@@ -384,8 +387,8 @@ def _detect_amd_gfx_codes() -> list[str]:
 _rocm_windows_torch_installed: bool = False
 
 
-def _install_bnb_windows_rocm() -> None:
-    """Install the AMD Windows BNB prerelease wheel.
+def _install_bnb_windows_rocm() -> bool:
+    """Install the AMD Windows BNB prerelease wheel. Returns True on success.
 
     The continuous-release wheel is intentionally mismatched: the filename
     encodes version 1.33.7.preview (parsed as 1.33.7rc0 by PEP 440) while the
@@ -395,11 +398,11 @@ def _install_bnb_windows_rocm() -> None:
     """
     _bnb_win_url = _BNB_ROCM_PRERELEASE_URLS.get("win_amd64")
     if _bnb_win_url is None:
-        return
+        return False
     _prev = os.environ.get("UV_SKIP_WHEEL_FILENAME_CHECK")
     os.environ["UV_SKIP_WHEEL_FILENAME_CHECK"] = "1"
     try:
-        pip_install_try(
+        _ok = pip_install_try(
             "bitsandbytes (AMD Windows, pre-release main)",
             "--force-reinstall",
             "--no-cache-dir",
@@ -412,6 +415,8 @@ def _install_bnb_windows_rocm() -> None:
             os.environ.pop("UV_SKIP_WHEEL_FILENAME_CHECK", None)
         else:
             os.environ["UV_SKIP_WHEEL_FILENAME_CHECK"] = _prev
+    if not _ok:
+        return False
     # After install: detect the actual ROCm DLL suffix from the wheel so any
     # post-install BNB import in this process loads the correct DLL.
     # The worker subprocess does the same detection independently (worker.py §1f).
@@ -419,6 +424,7 @@ def _install_bnb_windows_rocm() -> None:
     if "BNB_ROCM_VERSION" not in os.environ:
         _ver = _detect_bnb_rocm_dll_ver() or "72"
         os.environ["BNB_ROCM_VERSION"] = _ver
+    return True
 
 
 def _ensure_rocm_torch() -> None:
@@ -431,14 +437,38 @@ def _ensure_rocm_torch() -> None:
     Uses pip_install() to respect uv, constraints, and --python targeting.
     """
     global _rocm_windows_torch_installed
-    # setup.ps1 sets this when it already installed AMD wheels; skip the probe.
+    # setup.ps1 sets this when it already installed AMD wheels; skip the probe
+    # only when torch is actually importable as ROCm. If the venv was wiped
+    # between runs, the stale env-var would suppress a needed reinstall.
     if os.environ.get("UNSLOTH_ROCM_TORCH_INSTALLED") == "1":
-        _rocm_windows_torch_installed = True
-        # setup.ps1 already installed ROCm torch, but we still need to install
-        # the AMD Windows BNB wheel here — the PyPI bitsandbytes wheel ships
-        # only CUDA DLLs and will fail to load on ROCm (no libbitsandbytes_rocm72.dll).
-        _install_bnb_windows_rocm()
-        return
+        _torch_ok = False
+        try:
+            _probe = subprocess.run(
+                [
+                    sys.executable,
+                    "-c",
+                    (
+                        "import torch; "
+                        "hip=getattr(torch.version,'hip','') or ''; "
+                        "import sys; "
+                        "sys.exit(0 if (hip or 'rocm' in torch.__version__.lower()) else 1)"
+                    ),
+                ],
+                stdout = subprocess.DEVNULL,
+                stderr = subprocess.DEVNULL,
+                timeout = 30,
+            )
+            _torch_ok = _probe.returncode == 0
+        except (OSError, subprocess.TimeoutExpired):
+            pass
+        if _torch_ok:
+            _rocm_windows_torch_installed = True
+            # setup.ps1 already installed ROCm torch, but we still need to install
+            # the AMD Windows BNB wheel here -- the PyPI bitsandbytes wheel ships
+            # only CUDA DLLs and will fail to load on ROCm.
+            _install_bnb_windows_rocm()
+            return
+        # torch was wiped between runs; fall through to the full install path
     if IS_MACOS:
         return
 
@@ -488,11 +518,13 @@ def _ensure_rocm_torch() -> None:
                 "torchaudio",
                 constrain = False,
             )
-        # Always install AMD Windows bitsandbytes — the PyPI wheel ships only
+        # Always install AMD Windows bitsandbytes -- the PyPI wheel ships only
         # CUDA DLLs and will fail to load on ROCm.  Install even when torch was
         # already a ROCm build so that `studio update` repairs a broken bnb.
-        _install_bnb_windows_rocm()
-        _rocm_windows_torch_installed = True
+        # Only flip the success flag when the install actually succeeds; otherwise
+        # the post-install "manual install may be required" warning is suppressed.
+        if _install_bnb_windows_rocm():
+            _rocm_windows_torch_installed = True
         return
 
     # ── Linux x86_64 only: PyTorch ROCm wheels are not published for aarch64 ──