Add LinearFVFunctorRadiativeBC — second-order radiative heat flux BC for LinearFV

[deOliveira-R]

Summary

• Adds LinearFVFunctorRadiativeBC, the LinearFV counterpart of FVFunctorRadiativeBC.
• The nonlinear radiative flux $q = \sigma\varepsilon(T^4 - T_\infty^4)$ is Newton-linearized around the extrapolated boundary face temperature $T_{b,\text{old}} = T_P + \nabla T \cdot \mathbf{d}_{cf}$, yielding a Robin-type condition with second-order spatial accuracy.
• Inherits from LinearFVAdvectionDiffusionFunctorRobinBCBase; non-orthogonal corrections and all matrix/RHS contributions are handled by the Robin base class.
• Convergence to the nonlinear solution is driven by pseudo-transient Picard iteration.

Test plan

• Regression test: hot-body cooling converges to correct steady-state temperature distribution
• Limiting behavior: equilibrium case ($T_L = T_\infty$) produces uniform temperature and zero net flux
• MMS convergence test: quadratic manufactured solution with constant body force confirms $O(h^2)$ spatial accuracy (measured convergence rate: 2.005)

🤖 Generated with Claude Code

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Framework test summary

Compared against 8c713c9 in job civet.inl.gov/job/3611922.

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Modules test summary

Compared against 8c713c9 in job civet.inl.gov/job/3611922.

Added tests

Test	Time (s)	Memory (MB)
heat_transfer/test:linearfvbcs/linear_fv_functor_radiative_bc.second_order_convergence	4.20	0.00
heat_transfer/test:linearfvbcs/linear_fv_functor_radiative_bc.equilibrium	0.92	0.00
heat_transfer/test:linearfvbcs/linear_fv_functor_radiative_bc.hot_body_cooling	0.81	0.00

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