kernels: one shared unforced MRT collision for blocked, channel and patch paths

BlockStep, BlockChannelStep and refinement.cu patch_collide each carried
the same compute_rho_u -> omega clamp -> collide_mrt_no_force sequence.
collide_mrt_unforced (collision_mrt.cuh, __forceinline__) now holds it
once; the three sites call it. Same operations in the same order: no
numerical, ABI or SRT change. The host source contract now requires one
helper definition and exactly one call per file, and forbids the old
inline sequence.

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

tests/unit/test_refinement_mrt.py

@@ -37,8 +37,13 @@ class RefinementMRTHost(unittest.TestCase):
             self.assertIn("STORE_PRECISION != 0", source)
             self.assertIn('#if COLLISION_MODEL == 2\n#include "operators/collision_mrt.cuh"\n#endif', source)
         self.assertIn("USE_DDF_SHIFTING ||", refinement)  # dense/mixed shifted stays rejected
+        # One shared unforced MRT body; each blocked/patch path calls it exactly once.
+        mrt = (KERNELS / "operators/collision_mrt.cuh").read_text()
+        self.assertEqual(mrt.count("void collide_mrt_unforced("), 1)
         for source in (blocks, channel, refinement):
-            self.assertEqual(source.count("collide_mrt_no_force("), 1)
+            self.assertEqual(source.count("collide_mrt_unforced(f);"), 1)
+            self.assertNotIn("collide_mrt_no_force(", source)
+            self.assertNotIn("compute_rho_u(f, rho, ux, uy);\n            collide_mrt", source)
 
 
 if __name__ == "__main__": unittest.main()