Fix GRCBC subsonic inflow using wrong L index

In the GRCBC subsonic inflow loop (do i = 2, momxb), L(2) was
hardcoded instead of L(i), causing only the second wave amplitude
to be updated rather than each wave amplitude in the loop.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_cbc.fpp

@@ -930,7 +930,7 @@ contains
                             if (bc_${XYZ}$%grcbc_in) then
                                 $:GPU_LOOP(parallelism='[seq]')
                                 do i = 2, momxb
-                                    L(2) = c**3._wp*Ma*(alpha_rho(i - 1) - alpha_rho_in(i - 1, ${CBC_DIR}$))/Del_in(${CBC_DIR}$) - c*Ma*(pres - pres_in(${CBC_DIR}$))/Del_in(${CBC_DIR}$)
+                                    L(i) = c**3._wp*Ma*(alpha_rho(i - 1) - alpha_rho_in(i - 1, ${CBC_DIR}$))/Del_in(${CBC_DIR}$) - c*Ma*(pres - pres_in(${CBC_DIR}$))/Del_in(${CBC_DIR}$)
                                 end do
                                 if (n > 0) then
                                     L(momxb + 1) = c*Ma*(vel(dir_idx(2)) - vel_in(${CBC_DIR}$, dir_idx(2)))/Del_in(${CBC_DIR}$)