Add ASSERT for psi_idx and clarifying comment for IB loop scope

src/pre_process/m_start_up.fpp:
- Add @:ASSERT(psi_idx > 0) before hyper_cleaning block to catch
  misconfigured cases where hyper_cleaning is enabled but psi_idx is unset

src/pre_process/m_mpi_proxy.fpp:
- Add comment explaining why IB patch broadcasts are in the num_patches_max
  loop rather than the num_bc_patches_max loop (patch_ib is sized by
  num_patches_max, not num_bc_patches_max)

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

Author: sbryngelson

src/pre_process/m_mpi_proxy.fpp

@@ -116,7 +116,8 @@ contains
             if (chemistry) then
                 call MPI_BCAST(patch_icpp(i)%Y, size(patch_icpp(i)%Y), mpi_p, 0, MPI_COMM_WORLD, ierr)
             end if
-            ! Broadcast IB variables
+            ! Broadcast IB variables: patch_ib is indexed 1:num_patches_max,
+            ! not 1:num_bc_patches_max, so these must live in the num_patches_max loop.
             #:for VAR in ['vel', 'angular_vel', 'angles']
                 call MPI_BCAST(patch_ib(i)%${VAR}$, size(patch_ib(i)%${VAR}$), mpi_p, 0, MPI_COMM_WORLD, ierr)
             #:endfor

src/pre_process/m_start_up.fpp

@@ -788,6 +788,7 @@ contains
 
         ! hard-coded psi
         if (hyper_cleaning) then
+            @:ASSERT(psi_idx > 0, "hyper_cleaning requires psi_idx to be set")
             do l = 0, p
                 do k = 0, n
                     do j = 0, m