Fix acoustic source and body force timing

Move mytime update to after the TVD RK call so that sub-step source
terms see the correct time. Also change acoustic source to use mytime
instead of t_step*dt, which was inconsistent with adaptive time stepping.

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

Author: sbryngelson

src/simulation/m_acoustic_src.fpp

@@ -165,7 +165,7 @@ contains
 
         integer, parameter :: mass_label = 1, mom_label = 2
 
-        sim_time = t_step*dt
+        sim_time = mytime
 
         $:GPU_PARALLEL_LOOP(private='[j,k,l]', collapse=3)
         do l = 0, p

src/simulation/m_start_up.fpp

@@ -845,13 +845,13 @@ contains
             end do
         end if
 
-        mytime = mytime + dt
-
         ! Total-variation-diminishing (TVD) Runge-Kutta (RK) time-steppers
         if (any(time_stepper == (/1, 2, 3/))) then
             call s_tvd_rk(t_step, time_avg, time_stepper)
         end if
 
+        mytime = mytime + dt
+
         if (relax) call s_infinite_relaxation_k(q_cons_ts(1)%vf)
 
         ! Time-stepping loop controls