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 <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/features.md

@@ -5,7 +5,7 @@ Generated from the capability catalogue (`crates/engine/src/catalogue.rs`) by
 Every entry is a toolbar button. **Ready** entries run their command; **coming soon** entries are
 shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 
-42 ready, 58 coming soon.
+45 ready, 55 coming soon.
 
 ## Geometry
 
@@ -90,8 +90,8 @@ shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 |---|---|---|---|
 | Boundary Conditions | Boundary Condition — Fixed, displacement or symmetry condition on a set or reference point | ready | `model.boundary_condition.create`, `model.boundary_condition.edit`, `model.boundary_condition.delete` |
 | Loads | Load — Force, moment, pressure, traction, gravity or body force | ready | `model.load.create`, `model.load.edit`, `model.load.delete` |
-| Flow | Fluid Domain — The fluid grid voxelised from the face-tagged surface | coming soon (M12) |  |
-| Flow | Inlet & Outlet — Velocity inlets, pressure outlets and walls | coming soon (M12) |  |
+| Flow | Fluid Domain — The box of fluid cells, its fluid, and the obstacle bodies voxelised from their face-tagged surfaces | ready | `model.fluid_domain.create`, `model.fluid_domain.edit`, `model.fluid_domain.delete` |
+| Flow | Inlet & Outlet — Velocity inlets, pressure outlets, walls, moving and slip walls, periodic faces | ready | `model.fluid_domain.boundary` |
 
 ## Interactions
 
@@ -123,7 +123,7 @@ shown greyed with the roadmap milestone that brings them (ROADMAP.md).
 | Implicit | Frequency — Natural frequencies and mode shapes of a frequency step | ready | `solve.frequency` |
 | Explicit | Explicit — Transient dynamics of an explicit step by central differences (contact and erosion coming in M10) | ready | `solve.explicit` |
 | Explicit | Blast & Impact — EOS-driven fluids with fluid–structure contact (ALE or SPH) | coming soon (M14) |  |
-| Fluids | Lattice Boltzmann — Lattice Boltzmann flow on the CPU or the GPU | coming soon (M12) |  |
+| Fluids | Lattice Boltzmann — Lattice Boltzmann flow of a fluid step on the CPU, with wall forces | ready | `solve.fluid` |
 | Fluids | Fluid–Structure — Coupled fluid and structure solve | coming soon (M13) |  |
 
 ## Results