Change only starter/report_tool.py. Standard library only; no new dependencies.

• load_records(path) -> list[dict]:
  • .csv files load through csv.DictReader.
  • .json files load as a list of objects; a non-list or non-object payload raises ValueError.
  • Any other suffix raises ValueError naming the unsupported file.
• build_report(records, *, required_fields, numeric_field, group_field) -> dict:
  • Returns {"total", "valid", "invalid", "groups", "errors"}.
  • Invalid rows never abort the run: each is collected in errors as {"index": <row position>, "reason": <short string>}. Invalid means: not a dict, missing or blank required field, or a non-numeric numeric_field value (booleans do not count as numbers).
  • Valid rows aggregate into groups[group_value] = {"count": int, "total": float}; each group total is rounded to two decimals.
  • total = valid + invalid must hold for every input.
• write_report(report, destination):
  • Writes JSON (UTF-8, indent 2, trailing newline) atomically: write a sibling temp file, then os.replace.
  • Creates missing parent directories.
  • Never leaves a .tmp file behind on success.
• run_scenario(scenario_dir) -> dict:
  • Reads scenario.json (data_file, required_fields, numeric_field, group_field, report_file), processes the data file, writes the report inside the scenario directory, and returns the report.
• main(argv=None) -> int:
  • Exactly one argument (the scenario directory). Otherwise print usage to stderr and return 2.
  • On success print total=... valid=... invalid=... to stdout and return 0.
  • On input failure print error: ... to stderr and return 1.

Done means python verify.py starter exits 0 with all nine tests passing, and python verify.py starter --expect-failure exits nonzero because the starter no longer reproduces the unfinished state.