rtx-cfd: TVD convection in the embedded predictor — the wake sheds — and Turek–Hron CFD2/CFD3

First-order upwind's numerical viscosity |u| h / 2 is ~10x the physical
viscosity on the Turek–Hron grids: the effective Reynolds number lands
near 20 and CFD3 (Re 200) produced NO vortex shedding at all — one lift
zero-crossing in three seconds at h = 10 mm. The physics, not a bug.

EmbeddedParameters gains `convection_scheme` (default Upwind, bit-
identical — the no-body degeneracy test still reads 0.0): the TVD branch
adds SIMPLE's limited face corrections (van Albada / van Leer,
`face_correction` now pub(crate)) directly in the explicit predictor —
no deferred iteration needed in an explicit step. Domain-side faces and
faces whose far-upwind node is outside fall back to upwind exactly as in
SIMPLE; near the body the stencil reads ghost values, which encode the
wall. Verified: the embedded-circle MMS error drops 10–16x below upwind
(8.16e-4 vs 8.49e-3 at n = 32) at observed order 1.56 (SIMPLE's TVD
measured 1.59–1.84).

tests/turek_hron_cfd23.rs — CFD2 (Re 100, steady) and CFD3 (Re 200,
periodic), both with the benchmark's inflow ramp, both measured as time
statistics over a window (never a snapshot), surface route primary and
the control volume printed as the diagnostic (its central-difference
evaluation truncation grows with the convective flux: the routes agree
to 0.6% at Re 20 and differ 15–25% at Re 100–200 on these grids).

Measured across h = 10 / 6.6 / 5 mm:
- CFD3 shedding frequency 4.2746 / 4.3400 / 4.3939 Hz vs the reference
  4.3956 — converging −2.8% -> −1.3% -> −0.04%;
- CFD3 lift mean −184 / +160 / −2.6 vs −11.9 — lands on the reference;
  lift amplitude ±438 / ±556 / ±557 vs ±437.8 — +27% at the finer grids,
  unconverged (the flag is 2/3/4 cells thick);
- CFD2 control-volume drag 152.4 / 143.3 / 139.4 vs 136.700 — +2.0% at
  5 mm; CFD2 surface drag sits ~−10% (the boundary layer is ~one cell);
  CFD2 lift −3.4 / +30.2 / +8.4 vs 10.53.
Suite defaults run CFD2 at ny = 62 and CFD3 at ny = 41 (cost); the
asserted bands are the measured ones (frequency 10%, mean drag 15%,
amplitude 35%), not accuracy claims; RTX_CFD2_NY / RTX_CFD3_NY run the
studies.

Also recorded: the CFD1 refinement study extended to h = 3.3 mm
(RTX_CFD1_NY): control-volume drag 14.8996 (+4.25%), apparent order
~0.70 sustained over four grids, control-volume lift 1.1332 vs 1.11905
(+1.3%).

rtx-cfd 318 -> 321 green (full suite 321 passed / 0 failed).

Co-Authored-By: Claude Fable 5 <[email protected]>

crates/specialized/rtx-cfd/tests/turek_hron_cfd.rs

@@ -119,6 +119,7 @@ async fn run_cfd1(ny: usize) -> CfdResult<Cfd1> {
             top: SideBoundary::Velocity,
         },
         poisson_solver: PoissonSolverKind::Multigrid,
+        ..EmbeddedParameters::default()
     };
     let mut solver = EmbeddedPisoSolver::new(config, params)?;
     solver.set_boundary_velocity(|x, y, _| {