systemtests: archive and compare iterations.log for implicit coupling

[AdityaGupta716]

Archives precice-*-iterations.log files from case directories into
iterations-logs/ on every run, and compares them by SHA-256 hash against
a reference sidecar (.iterations-hashes.json) to catch implicit-coupling
regressions.

What this does

• _collect_iterations_logs() finds all precice-*-iterations.log files across each case directory
• __archive_iterations_logs() copies them into iterations-logs/, prefixing with the case name when multiple cases are present to avoid filename collisions
• __compare_iterations_hashes() reads the reference sidecar and fails fast on any hash mismatch, missing log, or unexpected extra log
• Hash check runs before fieldcompare in run() so regressions are caught early with a clear message
• run_for_reference_results() writes the .iterations-hashes.json sidecar automatically alongside the reference tar.gz at reference generation time

Backward compatibility

If no .iterations-hashes.json sidecar exists for a test, the hash check is skipped and the test runs as before. Existing tests are unaffected until reference results are regenerated.

Closes #440

[AdityaGupta716]

hi @MakisH plz review