IB markers: index each save by the save count under CFL time stepping

[sbryngelson]

Summary

With CFL-based time stepping (cfl_adap_dt or cfl_const_dt) and parallel_io = T (shared file, not file_per_process), post_process shows the wrong immersed-boundary markers for moving IBs: every save in a block of 100 shows the markers of that block's last save.

The simulation writes each save's ib_markers into restart_data/lustre_ib.dat at an offset set by the save number, computed as time_step/t_step_save, and post_process reads it back the same way. Under CFL stepping the caller already passes the save count (save_count = int(mytime/t_save)), and t_step_save keeps its default of −100. So k/(-100) is 0 for saves 1–99, −1 for 100–199, and so on: saves overwrite one slot. Fixed-dt runs are unaffected.

Change

• f_save_index(t_step, cfl_mode, t_step_save) in m_helper_basic returns t_step under CFL stepping, else t_step/t_step_save. The writer (s_write_parallel_ib_data) and the reader (s_read_ib_data_files) both use it, so the two cannot drift apart.
• The file layout is unchanged, so existing fixed-dt output still reads.

Verification

A 2D moving cylinder, 100×100, cfl_adap_dt = T, 8 saves, parallel_io = T, 2 ranks. For each save, the centroid of the post_process ib_markers field was compared with the IB centre in restart_data/ib_state_<save>.dat.

	Marker centroid error vs IB centre
master	saves 1–7 all show save 7's markers; errors of 4.24, 3.52, 2.80, 2.08 and 1.36 cells at saves 1–5
this PR	≤ 0.17 cells at every save

Found while rendering a 201-save shock–particle-curtain run with 38 moving reacting particles, where markers at save 40 sat 0.5 mm from the particles.

./mfc.sh test -o IBM: 61 passed. No regression test is added: the suite has no pattern for checking post_process output against IB state.

The bug was found and verified as described above.

Acknowledgement

• I confirm this PR meets the above expectations and reflects my own understanding and real-world context.

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Consolidated into #1960. The source changes are retained as separate commits, and this original description is reproduced in full there. Original discussion remains available here.

[github-actions]

Lines of Code

File	Lines	Diff
src/common/m_helper_basic.fpp	123	+5
src/post_process/m_data_input.f90	440	+1

Directory	Lines	Diff
common	10453	+5
post_process	3500	+1
total	47432	+6