Lattice Boltzmann flow on the CPU: model, solver, engine (M12) (#36)

Model: Newtonian fluid materials; fluid domains (box, spacing, conditions on
the box faces, obstacle bodies with conditions on their named faces, initial
velocity, body acceleration); fluid steps with caps. Deck writers refuse
fluid steps; a fluid-only model needs no mesh in model.check; one step
selector (Model::select_step) for every solver.

New L2 crate modelcraft-lbm: D3Q19 two-relaxation-time collision with Guo
forcing; halfway bounce-back walls (moving walls after Ladd), slip faces,
periodic pairs; Zou–He velocity and pressure faces with the D3Q19
transverse correction; obstacles voxelised from the face-tagged boundary
surface with exact predicates and symbolic perturbation; momentum-exchange
forces per box face and per B-rep face; unit conversion, automatic time step
and Mach/tau limits; bounded runs with frames and force histories.

Engine: model.fluid_domain.create/edit/delete, model.fluid_domain.boundary,
solve.fluid (obstacles from the open design or imported surfaces, lowerable
caps), solve.probe for fluid fields and wall force histories. Fluid Domain,
Inlet & Outlet and Lattice Boltzmann toolbar entries are ready.

Verification (analytic): plane Poiseuille exact with Λ = 3/16 and second
order (2.000) with Λ = 1/4; Couette exact with the wall shear; Taylor–Green
decay second order (2.01, 2.00); staircase pipe; Stokes drag of a sphere
array within 1.7% of Hasimoto at R = 6; Zou–He channel; mass conserved to
round-off; bit-for-bit reproducibility. Benchmark: Schäfer–Turek 2D-1.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/catalogue.rs

@@ -261,8 +261,21 @@ pub static CATALOGUE: &[Capability] = &[
         "Force, moment, pressure, traction, gravity or body force",
         &["model.load.edit", "model.load.delete"],
     ),
-    soon("flow.domain", "Fluid Domain", Loads, "Flow", "M12", "The fluid grid voxelised from the face-tagged surface"),
-    soon("flow.boundary", "Inlet & Outlet", Loads, "Flow", "M12", "Velocity inlets, pressure outlets and walls"),
+    ready_more(
+        "model.fluid_domain.create",
+        "Fluid Domain",
+        Loads,
+        "Flow",
+        "The box of fluid cells, its fluid, and the obstacle bodies voxelised from their face-tagged surfaces",
+        &["model.fluid_domain.edit", "model.fluid_domain.delete"],
+    ),
+    ready(
+        "model.fluid_domain.boundary",
+        "Inlet & Outlet",
+        Loads,
+        "Flow",
+        "Velocity inlets, pressure outlets, walls, moving and slip walls, periodic faces",
+    ),
     // ---- Interactions ----
     ready_more(
         "model.reference_point.create",
@@ -306,7 +319,7 @@ pub static CATALOGUE: &[Capability] = &[
         "Transient dynamics of an explicit step by central differences (contact and erosion coming in M10)",
     ),
     soon("solve.blast", "Blast & Impact", Solve, "Explicit", "M14", "EOS-driven fluids with fluid–structure contact (ALE or SPH)"),
-    soon("solve.flow", "Lattice Boltzmann", Solve, "Fluids", "M12", "Lattice Boltzmann flow on the CPU or the GPU"),
+    ready("solve.fluid", "Lattice Boltzmann", Solve, "Fluids", "Lattice Boltzmann flow of a fluid step on the CPU, with wall forces"),
     soon("solve.fsi", "Fluid–Structure", Solve, "Fluids", "M13", "Coupled fluid and structure solve"),
     // ---- Results ----
     ready_more("results.deformed", "Deformed", Results, "Display", "The deformed shape of a solved step, to scale", &["results.clear"]),