refinement: remove the dense refinement route; level_blocks is required (D2)

Deleted: kernels/refinement.cu and step/patch_step.cu (PatchExplode,
PatchFineStep, PatchCoarseStep, the CELERIS_REFINEMENT_AUDIT counters);
in FixedPatchStepper the dense branches of _build_level/_build_pair/_hop,
_apply_fine_boundary, the fine-level CurvedLinkSoA wiring (_fine_curved,
_fine_flags, _curved, _link_tables, its set_jet_amplitude/initialize/close
paths) and the now-vacuous 'requires all-blocked' guards (shifted,
projected, channel, raw exchange, several L1 rectangles, depth >= 3).

level_blocks is required: one tile (by, bx) of positive integers per level,
else ValueError before any device work naming the whole-level tile
((ny, nx) at the root, the fine window shape for one rectangle) as always
valid. No automatic tile choice. Removed route: None in level_blocks
(dense or mixed dense/blocked storage); parity of D/D, D/B and B/B is
recorded at 1573942 and 2469af7.

Kept: the uniform-grid solver (uniform.cu, Simulation, CurvedLinkSoA,
CurvedBoundaryKernel), FixedPatch2D, pinned historical runners.
Docs: compiler entries, BlockExplode comment, lifecycle, supported_settings.

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

docs/supported_settings.md

@@ -10,12 +10,13 @@ measured; they are not general bounds.
 
 | Setting | Supported | Guard |
 |---|---|---|
-| Dimension / lattice | 2D D2Q9 only (3D refinement is future work) | `lbm/refinement.py` `FixedPatch2D.from_config`; `#error` guards in `lbm/kernels/blocks.cu` and `lbm/kernels/refinement.cu` |
+| Dimension / lattice | 2D D2Q9 only (3D refinement is future work) | `lbm/refinement.py` `FixedPatch2D.from_config`; `#error` guards in `lbm/kernels/blocked.cu` |
 | Precision / streaming | FP32 storage, double buffer | same guards |
 | Collision on refinement levels | SRT or MRT; TRT and LES rejected | `lbm/refinement.py` `FixedPatch2D.from_config` (re-checked for fine boundaries in `prepare_fine_boundary`) |
-| Shifted (ddf_shifting) storage | only when every level is blocked; dense or mixed rejected | `lbm/refinement_stepper.py` `FixedPatchStepper.__init__`, `FixedPatchStepper._check_storage`; `#error` guard in `lbm/kernels/refinement.cu` (dense patch kernel unshifted only) |
+| Shifted (ddf_shifting) storage | supported (every level is blocked); one representation on all levels | `lbm/refinement_stepper.py` `FixedPatchStepper._check_storage` |
 | Fine relaxation | 1/tau_fine must lie inside the configured omega guards | `lbm/refinement.py` `FixedPatch2D.from_config` |
 | Single blocked level `FixedPatchStepper(refinements=[], level_blocks=[(by, bx)], level_objects=[circles])` (replaces `PeriodicBlockStepper`, removed in 8e1feff) | doubly periodic, SRT only, no LES/Jets/projection/exchange observation, stationary passive circles; `download()` returns a 1-tuple | `lbm/refinement_stepper.py` `_check_levels`, `FixedPatchStepper.__init__`; `lbm/blocks.py` `static_circle_tables` |
+| Refinement storage | blocked on every level: `level_blocks` is required, one tile `(by, bx)` per level dividing that level; a whole-level tile (`(ny, nx)` at the root, the fine window shape for one rectangle) always divides. Dense and mixed dense/blocked storage were removed (parity of D/D, D/B and B/B recorded at 1573942 and 2469af7) | `lbm/refinement_stepper.py` `_check_levels`, `FixedPatchStepper.__init__` |
 
 Tests: `tests/unit/test_refinement_mrt.py`, `tests/validation/multigrid/test_refinement_mrt_gpu.py`,
 `tests/unit/test_shifted_storage.py`, `tests/unit/test_shifted_block_operators.py`.