Fetch UnsatCoreTAC files via unsat_core_map.json instead of the whole archive

[jar-ben]

Uses the job's unsat_core_map.json ({ ruleId -> [UnsatCoreTAC .txt] }) to fetch only
the referenced files via the per-file endpoint, instead of pulling the full output tar.

• fetch_output_file(job, rel_path) — fetch one Reports/-relative file via the /f/<name>
  endpoint (added to all fetchers). fetch_statsdata now delegates to it.
• unsat_core_map / unsat_core_filenames / read_unsat_cores — look up a rule's unsat-core
  .txt files (and their contents) by its treeView ruleId.
• extract_unsat_core_files — fetches only the mapped files; falls back to the output tar when
  the map is absent.
• CheckResult.rule_id (parsed from the treeView node) and NodeStatus.SANITY_FAILED.

Consumer flow: parse the treeView, select rules by status, then fetch their .txt by rule_id.
Works for any rule with unsat cores (verified, failed satisfy, or vacuous), not just sanity.

[shellygr]

lgtm, two memory related nits

[shellygr]

also - what is the rule_id?

[jar-ben]

also - what is the rule_id?

It's a unique rule identifier. We added it to the prover already some time ago, but it was not exported. There is a prover PR that exports it to treeView (which allows us to match treeView items (rule&method results) to corresponding unsat core files).