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/decks/src/lib.rs

@@ -83,6 +83,19 @@ pub fn with_steps(model: &Model, names: &[String]) -> Result<Model> {
     Ok(out)
 }
 
+/// A fluid step: solved by ModelCraft's lattice Boltzmann solver, never written to a deck.
+pub(crate) fn fluid_step(format: &'static str, step: &str) -> Error {
+    Error::Unsupported { format, what: format!("fluid step `{step}` (solve it with solve.fluid; write the structural steps only)") }
+}
+
+/// Every writer's first check: the model has no fluid step.
+pub(crate) fn refuse_fluid(format: &'static str, model: &Model) -> Result<()> {
+    match model.steps().iter().find(|s| s.procedure.fluid_domain().is_some()) {
+        Some(step) => Err(fluid_step(format, &step.name)),
+        None => Ok(()),
+    }
+}
+
 /// Every deck format, one row per writer.
 pub const FORMATS: &[Format] = &[
     Format { name: "abaqus", extension: "inp", writer: abaqus::write_resolved },