systemtests: archive fieldcompare diff files on failure

[AdityaGupta716]

What this PR does

When fieldcompare fails, the diff VTK files it generates (via the --diff flag already present in the compose template) are now copied to a diff-results/ folder inside the system test directory. This means they get included in the CI artifact and can be downloaded for visual inspection of exactly where results diverged.

Closes #441

Changes

• Added __archive_diff_files() to Systemtest.py — scans precice-exports/ for any *diff*.vtu, *diff*.vtk, or *diff*.vtp files after a fieldcompare failure and copies them to diff-results/
• Called only when fieldcompare_result.exit_code != 0 — no unnecessary copies on passing runs
• Updated README.md to mention that diff files are available in diff-results/ for ParaView inspection

Notes

The --diff flag is already passed to fieldcompare in docker-compose.field_compare.template.yaml, so the diff files are already being generated — they just weren't being saved anywhere accessible until now.

Checklist

• I added a summary of any user-facing changes in changelog-entries/742.md
• I will remember to squash-and-merge with a useful summary

[AdityaGupta716]

hi @MakisH, plz review! The --diff flag was already being passed to fieldcompare but the generated diff files were never saved anywhere accessible — this copies them to diff-results/ on failure so they show up in the CI artifact and can be opened in ParaView to see exactly where results diverged.

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/gsoc-2026-aditya-gupta/2773/4

[MakisH]

This PR seems to be building upon your previous one #744, making it difficult to merge independently.

It is also very similar to #740, which was opened first. Why this one as well?

[AdityaGupta716]

@MakisH This one searches recursively for diff files in subdirectories, matches all output formats fieldcompare produces not just VTK, has the safety check to avoid copy loops, and adds logging for the really common case where fieldcompare fails but doesn't end up writing any diff files at all.
yes the timeout and yaml changes don't belong here. I'll remove the unrelated stuff out of this PR right now if you'd prefer to go with this version.