feat: make the harness backend-agnostic

Everything about which accelerator a run targets was previously spelled CUDA:
device discovery went through nvidia-smi, pinning through CUDA_VISIBLE_DEVICES,
the vendor guard only recognised NVIDIA, and the -p plugin list was hardcoded in
run_pytest.sh with no way to drop the CUDA-only members. Running the same
operator corpus on another vendor's device therefore meant editing the drivers.

The selection now hangs off one ZL_BACKEND variable (cuda/musa/rocm/ascend, with
the usual aliases):

- backends/ owns per-backend device enumeration, visibility env keys and
  capability flags, so nothing above it names a vendor;
- plugin_plan.py derives the -p list from those capabilities instead of a
  hardcoded array, which is what lets a non-CUDA run omit the graph-capture and
  cluster-fix plugins rather than watch them fail;
- _timing_plugin selects the timer from the same capabilities: CUDA keeps
  graph capture with its per-shape fallback, other backends stay on upstream
  do_bench and record timer=direct, because capture-then-fallback on a device
  without graph support only produces noise;
- _device_setup_plugin replaces the NVIDIA-only vendor guard;
- providers/flaggems.py resolves the pytest node, so a caller may pass TEST_NODE
  directly instead of the OP/OP_FILE pair, and SHAPE_MODE=upstream can defer to
  FlagGems' own core_shapes;
- _run_identity.py writes run_identity.json next to each run, recording backend,
  device, timer, autotune mode and the resolved test node. Two runs whose
  calibers differ are otherwise indistinguishable after the fact.

run_batch gains --backend and --devices (--gpus kept as an alias) and carries
backend/device/timer into ops_status.csv. test_backends.py covers the registry,
alias resolution, visibility env and plugin planning.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backends/musa.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from .base import DeviceBackend
+
+
+class MusaBackend(DeviceBackend):
+    name = "musa"
+    visibility_env_keys = ("MUSA_VISIBLE_DEVICES",)
+    torch_namespace = "musa"
+    flaggems_vendor = "musa"
+    supports_cuda_graph = False
+    supports_mm_cluster_fix = False
+    supports_triton_hooks = True
+    # Device count comes from torch.musa or MUSA_VISIBLE_DEVICES. Do not guess
+    # a vendor CLI here; confirm the real listing tool before adding it.
+    list_command = None