Apply the QCR2000 correction to the turbulent stress only (fix #2738)

[bigboateng]

Proposed Changes

The QCR2000 quadratic constitutive relation (Spalart, Int. J. Heat and Fluid Flow, 2000) modifies the turbulent (Boussinesq) stresses, but CNumerics::AddQCR was applied to the total stress tensor at all call sites: laminar + eddy in the viscous fluxes (scalar and SIMD paths), and the purely laminar tensor in Friction_Forces, where the eddy viscosity is zero and the correction should vanish identically. For a pure-shear wall state this injects spurious normal stresses of ±0.6·τ_wall into the wall traction, which redistributes drag between the pressure and friction components while leaving total drag nearly unchanged — the exact signature reported in #2738.

This PR scales the correction by the turbulent fraction of the viscosity used to build the tensor, mu_t / (mu_l + mu_t), which recovers the turbulent-only correction exactly because the stress tensor is linear in the viscosity (SA carries no turbulent-kinetic-energy term). The wall-force site passes a zero fraction on smooth walls. A unit test covers both properties (zero correction at mu_t = 0; scaled-total ≡ separate-turbulent-tensor).

Validation (Ubuntu 24.04, source build at de8c5015, the #2738 reporter's Joukowski R2 grid, 12000 iters, rms[Rho] ≈ −11):

run	CDp	CDv	CD
SA	0.003180	0.004735	0.007915
SA-QCR2000, before	0.003482	0.004432	0.007914
SA-QCR2000, after	0.003183	0.004716	0.007898
SA on patched build	0.003179	0.004735	0.007915

Before the change, QCR2000 shifts CDp +9.5% / CDv −6.4%; after it, the split returns to within 0.1% of SA, consistent with CFL3D/FUN3D/Fluent behavior for this attached flow (HiFi CFD verification workshop references). With QCR disabled the patched build is behaviorally unchanged.

Note: the turb_SA_QCR_RAE2822 reference values in parallel_regression.py shift by 1e-4…4e-3 (verified on a same-platform before/after pair); I will update them from this PR's CI output, since reference values are runner-specific. Opening as a draft until then.

Related Work

Fixes #2738.

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.