Add Smagorinsky LES with wall damping

[ia267]

Following on from Incompact3d, start with static Smagorinsky + wall damping. Backend-agnostic module selectable from config.

Scope

• Strain-rate tensor S_ij from the resolved velocity and its magnitude |S|
• Smagorinsky eddy viscosity $\nu_t = (C_s \Delta)^2 |S|$, with the filter width $\Delta$ from the local mesh spacing
• Wall damping for nut $\nu_t$ near a wall - the Mason-Thomson mixing-length form (C0, n) = (0.14, 3) used in Deskos (2019) . This is what supresses the log-law overshoot and is required for the ABL validation to reproduce the curves from Deskos (2019)
• Add the SGS stress divergence $\partial_{x_j}(2 \nu_t S_{ij})$ to the momentum RHS (via forcings/transeq) on both OMP and CUDA backends
• Expose the computed nut field so the ABL wall model Add neutral ABL driver #317 can read it
• Config: LES model selector + Smagorinsky constant + wall-damping (C0, n) in src/config.f90

Validation

• Validate against TGV at Re=5000, full periodic on [0, 2pi]^3 using existing 'case_tgv_t`. At Re=5000 the flow is under-resolved.
• Diagonostic: the dissipation rate over the transition/breakdown $t \approx 0\text{-}20$. The enstrophy based form $\varepsilon(t) = 2\nu,E(t)$ is available directly. The kinetic-energy form $\varepsilon(t) = -dE_k/dt$ needs $E_k$ added to the monitoring output
• Reference: a well-resolved DNS $\varepsilon(t)$ curve
• Criterion: the coarse-mesh Smagorinsky LES reproduces DNS dissipation rate peak (timing + magnitude) within tolerance, where the coarse-mesh no-model does not - i.e. the SGS model demonstrably improves the under-resolved result toward the DNS

Acceptance criteria

• m_les is backend agnostic
• Smagorinsky nut + SGS stress divergence computed on both backends; unit test on a known strain field recovers the analytical nut
• Wall damping reduces nut to ~0 at the wall with the (0.14, 3) profile
• TGV Re=5000 LES reproduces the DNS dissipation-rate peak $\varepsilon(t)$ (timing + magnitude) within tolerance, improving on the coarse no-model run
• Eddy-viscosity field exposed for the ABL wall model