[codex] Preserve fractional rollout hard scores

[zixuanguo786-ctrl]

Summary

• Store RolloutResult.hard as a float instead of truncating through int().
• Preserve continuous hard rewards such as 0.75 through from_dict() and to_dict().
• Add a regression test covering fractional hard score round-tripping.

Test plan

• uv run --with pytest pytest tests/test_types.py tests/test_scoring.py -q
• uv run --with ruff ruff check skillopt/types.py tests/test_types.py

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