feat: enrich Runtime operation evidence

[hyeokjun32]

Summary

• Enriched additive Runtime operation evidence in result JSON.
• Added backend availability, latency-budget/deadline, tegrastats availability, severity, retry hint, and indexed runtime lifecycle event fields.
• Updated schema guards, smoke assertions, and Runtime docs without changing existing top-level Lab-compatible result fields.

Validation

• bash scripts/smoke_default.sh
• python3 tests/test_manifest_compare_identity.py
• python3 tests/test_lab_result_schema.py
• python3 tests/test_agent_runtime_result_contract.py
• python3 -m unittest discover -s tests -p 'test_*.py'

Notes

• Runtime remains an evidence exporter, not a scheduler or deployment decision owner.
• New fields are additive and backward-compatible for existing Runtime result consumers.