test(D1): exchange, Jet and fine-surface checks run on all-blocked storage

- test_exchange_window and test_raw_exchange_gpu fine(): dense root becomes
  a (12, 8) blocked root (fine(None) stays only for the dense reference test
  that D2 retires);
- jets: G1/G3 compare blocked layouts against the FINE_BLOCKS[0] reference
  (blocked root); the dense run is a separate leg D2 removes; G2 legacy
  actuation vs Jet on the blocked reference; CPU fixtures use a blocked root;
- test_refinement_blocked_fine.raw reads every level through its layout;
- test_fine_surface_gpu: new test_fine_surface_blocked_matches_uniform_four_hops
  (fine ROI after each of four hops vs the uniform 48^2 solver with the same
  tolerances, blowing/suction mass sign from raw exchange, zero actuation ==
  passive bit for bit); the cross-interface donor rejection passes tiles.
  The dense hook/ledger/audit probes in test_fine_surface_four_hops and the
  dense SoA fault tests stay until D2 retires them by name.

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

tests/validation/multigrid/test_raw_exchange_gpu.py

@@ -92,8 +92,9 @@ def census(run, index, buffer):
 
 
 def fine(block):
-    """Two-level circle+jet construction on BOUNDS; block None keeps the dense fine level."""
-    return dict(refinements=[BOUNDS], level_blocks=[None, block],
+    """Two-level circle+jet construction on BOUNDS, all-blocked with a (12, 8) root tile;
+    block None is the dense route (dense reference test only, retired with the dense route)."""
+    return dict(refinements=[BOUNDS], level_blocks=[None if block is None else (12, 8), block],
                 level_objects=[(), (circle(),)], level_jets=[(), (jet(),)])
 
 class RawExchangeGPU(unittest.TestCase):