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>

backends/base.py

@@ -90,6 +90,13 @@ class DeviceBackend:
 
     def denies_capture(self, op: str) -> bool:
         return bool(op) and op in self.capture_deny_ops
+
+    # Plugins that patch around upstream assumptions this backend violates, e.g.
+    # FlagGems benchmarks that gate themselves on torch.cuda.is_available() even
+    # though the operator under test runs on whatever device FlagGems dispatched
+    # to. Loaded first so collection-time overrides land before anything reads
+    # the item list. Each such plugin must scope itself to its own vendor.
+    compat_plugins: Tuple[str, ...] = ()
     # Optional listing command; (argv, line_prefix). prefix="" means count
     # non-empty lines. Must be a real vendor CLI; do not invent flags.
     list_command: Optional[Tuple[Sequence[str], str]] = None