Regrid_DGModel2D_t Subroutine

  subroutine Regrid_DGModel2D_t(this,mesh,geometry)
    !! Rebind a live model to a new mesh/geometry pair (AMR regrid). The mesh-sized solution
    !! storage is reallocated and the boundary-condition registrations and maps are rebuilt
    !! for the new mesh, while everything that is not mesh-sized is preserved: the time state
    !! (t, dt, entropy, IO counter), the time-integrator selection, configuration flags, and
    !! any model-specific parameters (Init is intent(out) and would reset all of these).
    !! nvar/nstepped are unchanged - the model solves the same equations on a new mesh.
    !!
    !! The solution interior is left UNINITIALIZED: the caller transfers the solution from the
    !! previous mesh (e.g. ApplyTransferPlan on a BuildTransferPlan mapping) and then calls
    !! solution%UpdateDevice. Regrid runs once per adaptation epoch, between time steps; it is
    !! not a per-step hot path.
    implicit none
    class(DGModel2D_t),intent(inout) :: this
    type(Mesh2D),intent(in),target :: mesh
    type(SEMQuad),intent(in),target :: geometry

    if(.not. associated(this%mesh)) then
      print*,__FILE__,':',__LINE__, &
        ' : Error : Regrid called on a model that has not been initialized.'
      stop 1
    endif

    ! Free everything sized by the old mesh, mirroring Free (AdditionalFree releases any
    ! model-specific mesh-sized state so AdditionalInit can rebuild it below).
    ! Boundary-condition registrations are rebuilt because the boundary side set changes with the
    ! mesh. The mesh-sized fields are NOT freed: they are resized in place below (AMR Stage 6b),
    ! which reuses their host pools and device buffers whenever the new element count fits.
    call this%hyperbolicBCs%Free()
    call this%parabolicBCs%Free()
    call this%AdditionalFree()

    ! Rebuild on the new mesh, mirroring the mesh-sized portion of Init.
    this%mesh => mesh
    this%geometry => geometry

    ! Resize rather than Free + Init. Init is intent(out), so it would reset the whole object,
    ! reallocate every array, zero it, reconstruct the equation parsers and - on GPU builds -
    ! upload the zeros, all of which the adaptive loop then discards. Profiling attributed over
    ! half of an adaptation to exactly that cycle.
    call this%solution%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%workSol%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%dSdt%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%solutionGradient%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%flux%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%source%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)
    call this%fluxDivergence%Resize(geometry%x%interp,this%nvar,this%mesh%nElem)

    call this%solution%AssociateGeometry(geometry)
    call this%solutionGradient%AssociateGeometry(geometry)
    call this%flux%AssociateGeometry(geometry)
    call this%fluxDivergence%AssociateGeometry(geometry)

    call this%hyperbolicBCs%Init()
    call this%parabolicBCs%Init()

    call this%AdditionalInit()

    call this%MapBoundaryConditions()

    call this%SetMetadata()

  endsubroutine Regrid_DGModel2D_t