feat(report): add events.jsonl SHA-256 to result_summary.json

[arekay-nv]

Summary

Adds a content SHA-256 of the run's events.jsonl to the benchmark report so a
run's raw event log is fingerprinted and verifiable.

• New field events_sha256 on the Report struct → serialized into
  performance/result_summary.json (alongside qps/tps/run_config).
• Content-only digest: hashes the file bytes only (streamed in 1 MiB chunks
  via the new utils/hashing.py::sha256_file), so copying or renaming the log
  never changes the value, and large event logs are not read into memory at once.
• null when no event log was produced (e.g. SIGKILL before the tmpfs salvage),
  so finalization never crashes on a missing log.
• Attached in _write_report_artifacts after the log is salvaged to
  report_dir, mirroring how per-dataset accuracy is attached post-hoc — keeping
  Report.from_snapshot a pure snapshot→Report function.

Changed files

File	Change
utils/hashing.py	New sha256_file(path, *, chunk_size) util (mirrors atomic_write.py)
metrics/report.py	New events_sha256: str | None field on Report
commands/benchmark/execute.py	Compute + attach digest in _write_report_artifacts
tests/unit/utils/test_hashing.py	Unit tests for the helper
tests/unit/metrics/test_report_builder.py	to_json round-trips the field

Testing

• sha256_file: content-only (copy/rename → same digest, matches plain
  hashlib), differs on content change, missing file → None, large streamed
  input.
• Report.to_json carries events_sha256 and defaults to null.
• Full report_builder + benchmark + benchmark_final_snapshot suites pass
  (no regressions); ruff/ruff-format/license headers clean.

🤖 Generated with Claude Code

[github-actions]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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