docs: add Runtime timeout observation fixture

[hyeokjun32]

Summary

• Added a committed Runtime result fixture for timeout observation evidence.
• Documented how timeout observation is handed off to Lab/AIGuard without claiming production request cancellation.
• Added schema coverage for the timeout-observed fixture.

Validation

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

Notes

• Runtime remains the evidence exporter and does not own deployment decisions.
• Lab can interpret this fixture as review-required context.