feat(fortran): Fortran interface can now use multi-zone

Fortran interface uses the new C api ability to call the naieve
multi-zone solver. This allows fortran calling code to make use of in
build parellaism for solving multiple zones

tests/extern/fortran/gridfire_evolve_single.f90

@@ -0,0 +1,93 @@
+program main
+    use iso_c_binding
+    use gridfire_mod
+    implicit none
+
+    type(GridFire) :: net
+    integer(c_int) :: ierr
+    integer :: i
+
+    ! --- 1. Define Species and Initial Conditions ---
+    ! Note: String lengths must match or exceed the longest name.
+    ! We pad with spaces, which 'trim' handles inside the module.
+    character(len=5), dimension(8) :: species_names = [ &
+            "H-1  ", &
+            "He-3 ", &
+            "He-4 ", &
+            "C-12 ", &
+            "N-14 ", &
+            "O-16 ", &
+            "Ne-20", &
+            "Mg-24"  &
+        ]
+
+    ! Initial Mass Fractions (converted to Molar Abundances Y = X/A)
+    ! Standard solar-ish composition
+    real(c_double), dimension(8) :: Y_in = [ &
+        0.702616602672027,     &
+        9.74791583949078e-06,  &
+        0.06895512307276903,   &
+        0.00025,               &
+        7.855418029399437e-05, &
+        0.0006014411598306529, &
+        8.103062886768109e-05, &
+        2.151340851063217e-05  &
+        ]
+
+    ! Output buffers
+    real(c_double), dimension(8) :: Y_out
+    real(c_double) :: energy_out, dedt, dedrho, snu_e_loss, snu_flux, dmass
+
+    ! Thermodynamic Conditions (Solar Core-ish)
+    real(c_double) :: T = 1.5e7      ! 15 Million K
+    real(c_double) :: rho = 150.0e0  ! 150 g/cm^3
+    real(c_double) :: tMax = 3.0e17  ! 10 Gyr total time
+    real(c_double) :: dt0 = 1e-12    ! Starting Timestep
+
+    ! --- 2. Initialize GridFire ---
+    print *, "Initializing GridFire..."
+    call net%gff_init(SINGLE_ZONE)
+
+    ! --- 3. Register Species ---
+    print *, "Registering species..."
+    call net%gff_register_species(species_names)
+
+    ! --- 4. Configure Engine & Solver ---
+    print *, "Setting up Main Sequence Policy..."
+    call net%gff_setup_policy("MAIN_SEQUENCE_POLICY", Y_in)
+
+    print *, "Setting up CVODE Solver..."
+    call net%gff_setup_solver("CVODE")
+
+    ! --- 5. Evolve ---
+    print *, "Evolving system (t = ", tMax, "s dt =", dt0, "s)..."
+    call net%gff_evolve(Y_in, T, rho, tMax, dt0, Y_out, energy_out, dedt, dedrho, snu_e_loss, snu_flux, dmass, ierr)
+
+    if (ierr /= 0) then
+        print *, "Evolution Failed with error code: ", ierr
+        print *, "Error Message: ", net%gff_get_last_error()
+        call net%gff_free() ! Always cleanup
+        stop
+    end if
+
+    ! --- 6. Report Results ---
+    print *, ""
+    print *, "--- Results ---"
+    print '(A, ES12.5, A)', "Energy Generation: ", energy_out, " erg/g/s"
+    print '(A, ES12.5)',    "dEps/dT:           ", dedt
+    print '(A, ES12.5)',    "dEps/drho:         ", dedrho
+    print '(A, ES12.5)',    "Neutrino Energy Loss:      ", snu_e_loss
+    print '(A, ES12.5)',    "Neutrino Flux:          ", snu_flux
+    print '(A, ES12.5)',    "Mass Change:       ", dmass
+
+    print *, ""
+    print *, "Abundances:"
+    do i = 1, size(species_names)
+        print '(A, " : ", ES12.5, " -> ", ES12.5)', &
+                trim(species_names(i)), Y_in(i), Y_out(i)
+    end do
+
+    ! --- 7. Cleanup ---
+    call net%gff_free()
+
+end program main