research: Dütsch config from the shipped config_lbm.json schema; real-loader test

The generated config failed the real loader (KeyError: cuda). It now copies
src/CelerisLab/configs/config_lbm.json and overrides only grid, physics,
collision/streaming/storage, y walls, LES off and uniform inlet profile;
inlet scheme and outlet mode stay the template's and are recorded in
case.json. New CPU test loads the generated file with load_lbm_config.

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

tests/unit/test_duetsch_protocol.py

@@ -85,3 +85,22 @@ def test_exact_point_inside_body_is_missing_even_with_fluid_corners():
     assert value is None and used == () and total == 0.0
     value, used, total = p.sample(field, fluid, 3.35, 3.0, centre, radius)  # just outside
     assert value is not None and total > 0.0
+
+
+def test_generated_config_passes_the_real_loader(tmp_path):
+    import json
+    import sys
+    sys.path.insert(0, str(Path(__file__).resolve().parents[2] / "research/duetsch_re100_kc5"))
+    try:
+        import run
+    finally:
+        sys.path.pop(0)
+    from CelerisLab.config import load_lbm_config
+    cfg_dict, u_max, n = run.config(20, 10.0, 2520)
+    path = tmp_path / "config_lbm.json"
+    path.write_text(json.dumps(cfg_dict))
+    cfg = load_lbm_config(str(path))
+    assert (cfg.nx, cfg.ny) == (n, n) == (200, 200)
+    assert cfg.collision == "MRT" and cfg.streaming == "double_buffer" and not cfg.ddf_shifting
+    assert cfg.velocity == 0.0 and cfg.viscosity == pytest.approx(u_max * 20 / 100)
+    assert cfg.y_wall_bc == "bounce_back"