Archive and compare iterations.log for implicit coupling (fixes #440)

[PranjalManhgaye]

summary

Implements #440: archive and compare precice-*-iterations.log files from implicit coupling runs, so we can detect regressions in coupling iterations.

changes

• tools/tests/systemtests/Systemtest.py: Collect precice-*-iterations.log into iterations-logs/ after each tutorial run and compare SHA-256 hashes against a reference sidecar. Fails the test when a sidecar exists and hashes differ.
• tools/tests/generate_reference_results.py: When generating references, write an .iterations-hashes.json sidecar next to each reference tar when iterations logs exist.
• tools/tests/README.md: Document this behaviour.

test (local)

notes

• Backwards compatible: tests without a sidecar skip iterations log comparison.
• Explicit coupling runs (no iterations.log) are unaffected.
• Structured to allow extending to other files later.

checklist

• Changelog entry added in changelog-entries/440.md
• Ready for squash-and-merge if preferred

[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-pranjal-manhgaye/2769/5