feat: absorb the MThreads compat plugin and the A/B range report

Three pieces from the fork's feature/musa-benchmark-support branch that never
reached main, rebased onto the backend abstraction rather than copied verbatim.

_musa_benchmark_compat_plugin drops FlagGems benchmark skip guards written as
`torch.cuda.is_available()` where the code under test dispatches through
flag_gems.device. On MThreads those turn a working operator into "not measured":
test_fft.py skipped entirely, and unskipped on an S5000 it produces 25 rows. The
plugin pins each override to one file and one exact skip reason and only acts
when flag_gems has dispatched to the mthreads vendor; two guards were probed on
device and deliberately left in place, which is why they are named in its
docstring rather than silently absent.

It is wired through a new DeviceBackend.compat_plugins rather than a name test
in the planner, so the policy stays where every other backend difference lives.
Compat plugins are planned first because they rewrite collection-time markers
and nothing may read the item list before them; a test pins that position and
another pins that they do not leak to other backends.

batch_summary gains --ab: the speedup column compares gems against torch, not
one build against another, so an A/B needs the two run.logs paired by operator,
dtype and size detail. Only rows that succeeded on both sides with a positive
latency count, so an incomplete pairing cannot look like a complete range, and
the range is annotated when either side carries a BENCHMARK_DIRECT_NO_CUDAGRAPH
marker, since a fallback row was timed under a different regime.

ab_rlc_test.sh is the same caliber as ab_fold_test.sh with the comparison axis
moved to FLAGTREE_<VENDOR>_RLC_ENHANCE. Both are caliber examples, not
regression scripts: if the switch under test no longer exists the script still
runs every round and prints a full table showing no difference, so confirm the
axis is live before reusing either.

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

_musa_benchmark_compat_plugin.py

@@ -0,0 +1,89 @@
+"""Narrow compatibility fixes for the frozen MUSA product corpus.
+
+Several upstream FlagGems tests gate themselves on ``torch.cuda.is_available()``
+even though the code under test runs on any accelerator FlagGems dispatches to.
+On MThreads those guards skip the test instead of exercising the operator, which
+turns a working path into a verdict of "not measured".
+
+Every override here is pinned to one file and one exact skip reason, applies
+only when FlagGems has dispatched to the MThreads vendor, and is only added
+after the unskipped test was observed to pass on the device.
+
+Two files were probed on S5000 and deliberately left out, because their guards
+are not stale:
+
+- ``test_cutlass_scaled_mm.py`` gates on vLLM as well as CUDA, so removing the
+  guard would raise on the missing dependency instead of measuring anything.
+- ``test_grid_sample.py`` builds its inputs through a helper whose device
+  argument defaults to the literal ``"cuda"``. Its guard is consistent with its
+  own body: unskipped, all 90 collected cases fail in ``aten::empty`` on a CUDA
+  backend that does not exist here. Making that file portable means editing
+  upstream test source, which is outside what a compatibility plugin may do.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+
+# file name -> exact skipif reasons that are stale on a dispatching backend.
+_STALE_CUDA_GUARDS: dict[str, frozenset[str]] = {
+    "test_fft.py": frozenset({"CUDA is not available"}),
+    "test_top_k_per_row_decode.py": frozenset({"CUDA device required"}),
+}
+
+# Guards that are only stale for one test in the file stay pinned to it.
+_PINNED_TESTS: dict[str, str] = {
+    "test_fft.py": "test_perf_fft",
+}
+
+
+def _is_stale_cuda_guard(item, mark) -> bool:
+    if mark.name != "skipif":
+        return False
+    name = Path(str(item.path)).name
+    reasons = _STALE_CUDA_GUARDS.get(name)
+    if reasons is None or mark.kwargs.get("reason") not in reasons:
+        return False
+    pinned = _PINNED_TESTS.get(name)
+    return pinned is None or item.name == pinned
+
+
+# Retained under its historical name: the sealed campaign's tests reference it.
+def _is_stale_fft_cuda_guard(item, mark) -> bool:
+    return Path(str(item.path)).name == "test_fft.py" and _is_stale_cuda_guard(
+        item, mark
+    )
+
+
+def pytest_collection_modifyitems(items):
+    import flag_gems
+
+    if flag_gems.vendor_name != "mthreads":
+        return
+    removed: dict[str, int] = {}
+    # A guard can sit on the test, its class, or the module's pytestmark, and
+    # skipif is resolved by walking that whole chain. Strip every level.
+    seen: set[int] = set()
+    for item in items:
+        for node in item.listchain():
+            if id(node) in seen:
+                continue
+            seen.add(id(node))
+            markers = getattr(node, "own_markers", None)
+            if not markers:
+                continue
+            kept = [mark for mark in markers if not _is_stale_cuda_guard(item, mark)]
+            dropped = len(markers) - len(kept)
+            if dropped:
+                markers[:] = kept
+                name = Path(str(item.path)).name
+                removed[name] = removed.get(name, 0) + dropped
+    for name, count in sorted(removed.items()):
+        print(
+            f"[musa-benchmark-compat] removed {count} stale CUDA-only skip "
+            f"guard(s) from {name}",
+            file=sys.stderr,
+            flush=True,
+        )