test: MRT refinement coverage and obsolete rejection fix

Remove MRT from the FixedPatch2D rejection matrix (TRT and other
rejections retained); convert host MRT tests to stdlib unittest; add GPU
gates for unshifted dense-vs-blocked MRT two-level roots (exact bits) and
a shifted all-blocked three-level MRT smoke checking per-level omega.
No numerical source change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/test_refinement.py

@@ -142,7 +142,7 @@ class FixedPatchTests(unittest.TestCase):
                 FixedPatch2D.from_config(LBMConfig(nx=nx, ny=ny, viscosity=.1), bounds)
 
     def test_unsupported_backend_and_fine_relaxation_rejected(self):
-        for kwargs in ({"collision": "TRT"}, {"collision": "MRT"}, {"streaming": "esopull"},
+        for kwargs in ({"collision": "TRT"}, {"streaming": "esopull"},
                        {"les_enabled": True},
                        {"store_precision": "FP16S"}, {"lattice_model": "D3Q19"},
                        {"data_type": "FP64"}, {"omega_min": 1.0}):