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/solver/tests/common/mod.rs

@@ -123,7 +123,7 @@ impl Grid {
     pub fn define(&mut self, elastic: Elastic, density: Option<f64>, formulation: Formulation, procedure: Procedure) {
         let all: Vec<u32> = self.model.elements().keys().copied().collect();
         self.model.add_set("all", Set { elements: all, ..Set::default() }).unwrap();
-        self.model.add_material("Mat", Material { elastic: Some(elastic), density }).unwrap();
+        self.model.add_material("Mat", Material { elastic: Some(elastic), density, fluid: None }).unwrap();
         self.model.add_section("solid", Section { set: "all".into(), material: "Mat".into(), kind: SectionKind::Solid {}, formulation }).unwrap();
         self.model.add_step(Step::new("Step", procedure), None).unwrap();
     }