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.