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>

test_backends.py

@@ -173,6 +173,17 @@ class PluginPlanTests(unittest.TestCase):
         self.assertNotIn("_cudagraph_plugin", names)
         self.assertNotIn("_mm_cluster_fix_plugin", names)
 
+    def test_musa_loads_its_compat_plugin_first(self):
+        names = self._plan("musa")
+        self.assertIn("_musa_benchmark_compat_plugin", names)
+        # It rewrites collection-time markers, so nothing may inspect the item
+        # list before it runs.
+        self.assertEqual(names[0], "_musa_benchmark_compat_plugin")
+
+    def test_compat_plugins_do_not_leak_to_other_backends(self):
+        for name in ("cuda", "rocm", "ascend"):
+            self.assertNotIn("_musa_benchmark_compat_plugin", self._plan(name), name)
+
     def test_musa_same_core_without_cluster_fix(self):
         # _cudagraph_plugin is absent from the plan on every backend: the timing
         # plugin loads it, so its absence here says nothing about capture support.